Self-identity is the mind's answer to the question "Who are you?"
Human beings are very, very similar. We are all composed from the same ratios of chemical constituents; we have only minor variations in height, weight, and colour; our anatomies all correspond. As physical constructs, we are only subtly unique. Yet we have the undeniable urge to establish ourselves as unique, as independent, with thousands of neat little idiosyncracies that define us as a person. We must be us. Be it socially obtained or biologically tattooed, one of any person's most motivating drives is the desire for identity.
It all begins at birth. You get a handful of interesting, esthetically-appealing names which are supposed describe you. Your parents dress you up in cute sailor suits and show you off to their neighbours. Sesame Street tells me that you are unique, just like everyone else. That you should be yourself, whatever that may be. But it doesn't really catch on yet. As children, we all somehow seem the same, a blended mix of goofy faces and ragged sweaters our parents forced us to wear to school.
It is not until we become teenage, though the age continually grows younger, that identity becomes an unquenchable thirst. Suddenly, you need to have friends, and not just any friends, but the right friends. Friends who are like you. You need to wear clothes, the right clothes. Clothes that are like you. You listen to different music. You believe in a different god. And you like to vocalize your difference from everyone else every chance you get. You enjoy filling out those little quizzes that define how you are into one of three absurdly broad categories. You read your horoscope, and squeal, "That is sooooo me!" You writhe with pleasure when someone assigns your name an adjective. You are drowning in an orgy of self-identity, and you can't get enough.
"What's your favourite kind of sushi?"
Later in life, the intelligent human tends to realize that their unique personality, carefully constructed of pop-culture magazine paper maché and cannibalized Gap clothing, is not serving them as well as they thought it would. It all seems so hollow to you. You've gone and bought your personality out of the bargain bin at K-Mart. You've lost yourself in a closet of Prada. You throw it all away; you spend a week living in your mother's basement drinking screwdrivers and eating KD. You have your identity crisis. Just like the rest of them.
Eventually, the human being must have a self-actualization, and dedicate themselves not to themselves, but to some outer meaning in the world. Or they might just go cuckoo. You join a church. Turn your daughter into a child beauty queen. Read lots of Douglas Coupland novels. Become a Jehovah's Witness and go about converting others. Node yourself mad with the glory of E2. You have a meaning, a calling, and you usually end up sticking with it until you die. Otherwise it might crumble again, leaving you a step backwards.
This is the fun, fun concept of self-identity. Isn't culture grand?
And in the end, we still don't know the answer.