Why? Why does everyone seem to have this need to find out what some random person thinks they are like based on a few multiple-choice questions? Why are we so pleased when we get an answer we like, or the same answer as a friend, and so annoyed when we get something that doesn't seem to fit? Is it related to the ever-expanding morass of self-help books in the bookstores, and the ever-present quizzes in women's magazines? Does it come down to some basic psychological drive? It might...
Humans, as social creatures, do seem to have a need to be able to label each other - parent, child, teacher, coworker, priest, politician, janitor. It provides a convenient way to encapsulate and retrieve information. It's much easier to think of a person as 'the janitor' than 'the person who cleans up the office after everyone's gone home, who's spanish, has a cute smile, and a faraway look in her eyes'.
This makes sense for people we don't know well, or people we don't interact with often - it's an efficient, if error-prone, way to store information about them. But why do we also delight in doing it to ourselves, and to those we are close to? I think it may be because it helps us fit ourselves (and our friends) into the larger puzzle of society. We do have a fairly hard-wired 'us and them' mentality that makes us seek out people like ourselves. If I'm a brown dragon, then my most comfortable place in society is doing brown-dragonish sorts of things, and interacting with other brown dragons.
I've always liked dissecting my own brain - figuring out why I think things, how my thought and emotional processes work. It's the biggest, most complicated jigsaw puzzle I've ever seen, it's right here whenever I feel like working on it, and gives instant feedback. Similarly for those of my friends. I've got my own system for 'personality pigeonholing' people (which I'll post about later). I have actually gained some insight into my own personality through it, in the same way I learn more about myself by reading books on ADD. It doesn't tell me qualities of my personality that I don't already know, but it reveals underlying reasons and larger patterns that I might not have seen before.
So it comes down to a basic drive to put labels to things, as an efficient means of information management, and a way to link yourself to the larger world in some meaningful way. It's also, in essence, a shortcut - our brains are lazy, and if we see a way to get somewhere without having to think as much about it we'll take it.
(Btw - if you made it through this, congratulations :). I'd like to put some version of this on my web page with my other writing, so questions/comments/suggestions of any sort are encouraged.)