You hate yourself
You hate everyone else
At least, this is the theory of a friend of mine. And it makes sense to an extent. Punk really came about with the Sex Pistols in the late 70's, at least this was the defining punk band to start the whole genre. It was about anger over politics, people, society, and in general breaking the rules.
Grunge, on the other hand, hit it big with Nirvana in the late 80's. Nirvana didn't want fame, didn't want people dressing like them, didn't want radio play, they didn't want much of anything. Kurt was of the opinion that they weren't that great, but they didn't care and the rest of the world could take a flying fuck. I think my friend is a bit off on this point, grunge seems more apathetic than self loathing to me.
I think either one tends to have a bit of the fuck the world attitude, it just boils down to why they feel that way. Of course now I'm generalizing which is never good...