If you adopt a specific social group, you are adopting their subculture. That doesn't mean people stay in one subculture all their life, that's totally wrong - so no, that's not "it". Look at all those punks, hippies, mods, etc, that are now the men and women in suits/"normal" clothes, looking like every other middle-aged person, you know? They exit the subculture and move into the culture. But you still see some middle-aged punks and that walking around sometimes.
Anyways, for example, a person who wears gothic clothes - although they may be a happy person - they are generally into morbid things (my best friend was a goth for a goth for a few years, books on depression, morbid looking comics - forgot what they were called, talks about death, etc...). I dunno a definition for goth, but that's what I find.
I've never known any social groups to fight (I actually often see emos and townies/chavs hang out together) - although I'm sure they do, but that sounds hella stupid.