Part of the motivation also seems to be that we've fetishized disorders; having or claiming to have disorder X is a weird way to feel "special" or exceptional. Another part of it is that it functions as a kind of instrument for uptight people to gain a sense of exemption from social norms - real or merely perceived - that they feel burdened by or live in terror of "violating", but who are incapable of or unwilling to ignore them, because what would people say! "Oh, I'm weird, because I have disorder X. Can't help it, sorry! Can't judge me, because then you, not me, are the asshole!"
Needless to say, it's not very rational.
Which is not to say various disordered conditions don't exist.