To fit in! Because, if you're different, then people will not like you.
Example: you're walking down the street, and you see somone wearing old-skool billabong clothes ( before billabong became international ).
Don't you automatically think that person is different, and therefore worse?
Moral of the story: Right now, "cool" is to be angsty, depressed, have frequent mood-swings at the drop of a hat, and listen to punk. So guess who is trying to be cool now?