Before we start, just a brief note that for all practical purposes, the terms "you" and "mind"; "self-awareness" and "consciousness", is all pretty interchangeable and synonymous.
SOME YOU, YOU, YOU TRUISMS
Assuming the Universe was created some 13.7 billion years ago, ‘you' that will come to pass were in a state of non-existence for that duration – then ‘you' undergo or underwent a phase transition to existence for roughly 100 years or so, then ‘you' will undergo another phase transition back to non-existence for another lot of billions maybe trillions of more years. What's the point? However, think of all those might-have-been self-aware entities that never were and never will come-to-be. These never-ever entities have had and will have a state of non-existence from alpha-to-omega. The question now is might your ever-so-brief phase transition be a positive or negative aspect of your overall alpha-to-omega state of non-existence? I guess only you can answer that philosophical point-of-view. Now you have to admit that there was nothing overly distressing or traumatic about your 13.7 billion year pre-natal non-existence. So, that being the case, there's unlikely to be anything overly distressing or traumatic about your post-existence non-existence. Rest assured it will be just as peaceful, quiet, stress and tax free.
I give you a state of "non-existence" rather than "no existence" pre-natal and post-death on the grounds that IMHO the fixed and absolute laws, principles and relationships of the newly formed cosmos – physics in other words – was established at the point of the Big Bang event and thus we have a deterministic clockwork Universe (as per Newton and Einstein) and where all subsequent events unfold according to plan (via those absolute laws, principles and relationships).
You had no say in being you. You had no free will in the matter, which is a bit unfair; a bit of a bummer in that you were drafted into being you. You are all alone trapped inside your own mind, from which, excepting death, there is no escape from. So who are you going to sue?
"You" is that which you call yourself "me", or "myself", or "I": indicators of your own self-awareness.
I am to you "the other" and you are to me "the other". We share lots and lots of traits from basic biochemistry through to organs and organ systems up through basic body shape and structure plus a basic knowledge and understanding of the workings of life, the Universe and everything. We do not share any sense of our own sense of self-awareness. We each have our own unique sense of self. We can't get inside each other's head – which is probably a good thing. I have no comprehension what it is like to be you, or be my cat or the bird feeding outside the window at the bird feeder. So be it.
Your brain, and my brain, in fact all normal human brains, share one thing. The brain is the only organ capable of studying itself! Now that's profound!
WHY ARE YOU UNIQUE?
You are a one-off unique being among all humans who are and who have ever been; among all the animals who are and who have ever been. You are one shot in a trillion – then add in all those eggs and sperm, all those that might-have-been. But what really makes you unique?
What makes you, you? At first glance it would appear to be memory, experiences, knowledge. But you became you at conception at which time you had no memory, no experiences, or knowledge. So, scratch that idea.
Perhaps what makes you, you – your uniqueness – is the unique way your brain, hence your mind, is hardwired; that plus your unique brain chemistry.
Though you are unique, in theory there can only be a finite number of ways of wiring up the brain and the mind it contains. There could in theory have existed or could now exist another duplicate set of your brain wiring – that too must be you but you and that other you would never know (or even probably suspect), or would you, and your alter ego. After all, if there are an identical two of you out there, each should be experiencing what the other is experiencing as well as what they are experiencing. If that doesn't make your head hurt, nothing will!
But there has to be a connection between you at conception and you of today, be it 1, 10, 25 or 50 (plus) years on down the track. It's not brain wiring or brain chemistry either since at your conception there was no brain, hence no wiring, and no brain chemistry.
So what does that leave if it is not memories, experiences and knowledge nor hardwiring of your neurons and brain chemistry, etc.?
Aha – What makes you unique is that you are hardwired by your unchangeable DNA (except here your identical twin and/or clone would be you too with the same arguments that should have made your head hurt earlier).
DNA apart, a clone isn't you. DNA apart, an identical twin isn't you. Even if I were to use my home chemistry set and recreate your mind – atom by atom; molecule by molecule; neuron by neuron; and house the finished product in some sort of receptacle (brain-in-a-vat), it still wouldn't be you. You would still be you, separate and apart; as the you that is trapped inside of your own mind.
Yet, there are a finite number of combos of DNA, so again in theory there could have been, or currently is now, another you out there in la-la-land, but again you and that other you would never know (or even probably suspect), but then again, maybe not.
In any event, we can suggest that what makes you unique starts with your DNA at conception, then builds on that by adding hardwires and building your neural network as you grow up, culminating throughout the rest of your life by the accumulation of memories, experiences, and knowledge.
Yet, your body's, even your brain's atoms, molecules, entire cells are replaced on an ongoing basis. Why doesn't that alter who you are or your self-awareness? You were not the same being at age five as you are, were or will be at fifty, yet you are the same. Very mysterious!
What is crystal clear is that self-awareness or mind isn't something nebulous or intangible. It's all grounded in matter and energy. You have no self-awareness at conception. It is debatable if you have any self-awareness at birth. You have no self-awareness when you are naturally asleep or when you are artificially put to sleep – say for surgery. Like the surgery example, you can change your own state of self-awareness artificially, legally or illegally. You have no self-awareness in a coma and there are other medical conditions, especially in the elderly, where self-awareness isn't evident either.
WHY ARE YOU, YOU?
Presumably prior to your conception every other conception resulted in a not-you, that is, every other conception resulted in someone else (human or animal). Now that might not be the case since 1) you wouldn't remember if once upon a time you were a T-Rex or a Woolly Mammoth or a Neanderthal or Queen Cleopatra, or 2) an historical conception that would have been you naturally or spontaneously aborted before any memory could set in.
Presumably every conception that has eventuated since yours has been a not-you, or in other words someone else (human or animal). [Since plants don't have minds or self-awareness I'll eliminate them from consideration here.]
The question arises, why was your conception and your conception alone you and not a not-you, that is, why wasn't that conception too someone else (again, human or animal).
Presumably every conception after your demise will result in a not-you, or someone else (human or animal). But if there could have been a self-aware you in the past, then also presumably there could be another self-aware you in the future – I guess you'd call that reincarnation.
But a puzzle arises here. If there could have been you in the past, and/or you in the future, then what's preventing the conception of another you here and now? You would have to know, or so I suspect, otherwise you couldn't call that other you, you. In other words, it's apparently okay that there can be multiples of you in time, but not in space. Since that makes little sense, it's more logical to suggest that in the history of all conceptions, past, present and future, one and only one turned out to be you and all others were not-you, or rather someone else (human or animal).
I think it is safe to say that there's not another you in existence right now, otherwise you would know it by experiencing it.
You are your brain. You go where your brain goes. If I destroy your brain (while the rest of your body is kept artificially alive on life support), I destroy you and your self-awareness or consciousness. If I destroy your big toe, I do not destroy you and your self-awareness or consciousness. If your body is about to go kaput – now say it is riddled with cancer, and say someone else with a healthy body that is brain dead. Then if your brain is transplanted into that healthy but brain dead body, then that self-aware you goes along for the ride and you are the proud owner of a new body – sort of like moving house. You retain your self-aware you-ness. That would apply if your brain were transplanted into a machine (cybernetic organism) or even into that sci-fi staple, the brain-in-a-vat.
So where does the self-aware you go when the brain runs out of puff? The self-aware you will end up going where the brain goes, so that self-aware you too will run out of puff at the same time to decay or be pickled in a jar, or whatever. That also raises an interesting question – as I said above, if I destroy your brain I destroy you: but what if I destroy your brain by picking it off one cell at a time. Surely there would come a point where you would lose your sense of self-identity while still having some grey matter left intact.
Speaking of ‘what if' scenarios, it's not difficult to copy information, from the fax machine to the Xerox machine to making copies and backup copies of documents on your computer. It's easy to make copies of CDs and DVDs (legally and illegally). It's easy to download audio and video (legally and illegally). The question arises; eventually it should be possible to copy you, or your brain – all the information, memories, knowledge, experiences, etc. storied there – while leaving the original data in your grey matter intact. You would still be you in terms of that ‘you are your brain' observation, but would you now also be a ‘you are your software' housed inside some hardware inside some machine or computer? Would this be the ultimate self-aware dual-you that seemingly couldn't apply to you and your clone, or to you and your identical twin?
RESOLUTIONS & CONCLUSIONS
The entity that is you began at conception. That's the bedrock fundamental given. The only thing unique about you at conception was your DNA – I'm not aware of any chromosome or gene that has been identified as the "you" or "mind" or "self-awareness" chromosome or gene as opposed to the sex chromosome or a gene that gives you this or that trait. Your DNA is unique. It can point the finger at you in evidence if you commit a crime.
But that doesn't really work since you'd then also be you if you were cloned or you would share your self-awareness with your identical twin, and that's clearly not the case. Your clone or your identical twin has their own unique sense of self or identity.
Every conception has the potential to turn into identical twins. If it doesn't eventuate, out pops one unique being with that one self-awareness. If it does eventuate, out pops two unique beings each one with an unshared self-awareness. Very strange, except if the brain is you and there are two brains (one each for each twin) then there will be two separate-and-apart beings with self-awareness. The caveat is as long as the split into identical twins happens before the brain develops in the embryo then two separate-and-apart beings with self-awareness will result.
So what does that leave?
Barcodes! We're all familiar with barcodes. Each barcode or ISBN or ISSN or credit card number or social security number or account number or tax file number identifies a unique product or book or serial or credit card or social security recipient, etc. Now let's just suggest that each and every self-aware you that was, is and will be IS (not HAS) a barcode. Of course if you are just a one-off, that lone conception that resulted in you while all others resulted in a not-you, well the same applies. You have a barcode. Your clone would have a unique but different barcode; your identical twin ditto.
You can have more than one you or self-awareness per mind when you consider dual, split, multi-personality syndromes. But you cannot have one you in dual or multi minds. You don't wake up now and then again 12 hours later halfway around the world with another identity and another set of self-awareness. It's odd you can have the one but not the other. But perhaps you can (accidentally) be given more than one barcode though two individuals wouldn't be given the same barcode.
Two identical minds cannot view the same reality since they have different barcodes. From the moment of conception on, or at least from birth, identical minds will diverge into diverse separate and apart mindsets because of that.
That sort of applies in the inorganic world too. All non-living, inorganic bits, would have a unique barcode too – all electrons would share the same barcode; all grains of table salt, ditto, and so on. Yet, one grain of salt or one atom of oxygen can not experience the same sense-of-self (if that has any real meaning) as another grain of salt or another atom of oxygen. Since electrons and oxygen atoms and salt crystals have no self-awareness, it boils down to things that have the same barcode can have vastly different cosmic histories. Each electron is identical (same barcode), yet each electron has its own unique cosmic history. Be that as it may.
What's a barcode? A barcode is mathematics, or a mathematical number. That's also what computer software code is – a sequence of mathematical numbers (in this case in binary code).
So, my theory is that there is a software subroutine that's your barcode, and when that runs and interacts with other subroutines, well, that's you and that's your sense of identity and your self-awareness and your day-in-day-out life. When your subroutine starts running, that's your conception. When your subroutine terminates, you do too – that's your death.
Now that implies a software programmer but that doesn't of necessity translate into something supernatural – like God.
All this certainly explains why you have self-awareness while not having self-awareness of anything or anyone else. You may have a partner, children, siblings, etc. but their minds are as foreign to you as your mind is to them, just like your big left toe cannot know what it's like to be somebody else's big left toe.
I guess, by the way, calling you unique because you have a unique barcode, well to some people that barcode could be called a soul – a rose by any other name.
There is one other explanation. There only exists one self-awareness full-stop and that single consciousness is you. All else is a figment of your imagination or a creation of your dreams. Nothing really exists apart from you (or rather your mind) and you alone. If that's the case, I guess that makes you God!