Not one of those gangs that go around stabbing people, but just a cool-as-fuck crew of friends. I think I've spent my life chasing this and I hold the cartoons of my youth (and I mean pretty much all of them - Ghostbusters, TMNT, Captain Planet, Thundercats, etc) wholly responsible. 90% of the stuff you learn about life and relationships in those early, formative years is geared around a group of four or more rebels coming together for the greater good, and I think a lot of people don't leave that ideology behind. Certainly for men anyway.
Obviously a band is the ideal outlet for this desire, especially as everyone has clearly defined roles and an 'us against the world' attitude is kind of required as default, but I think any kind of close-knit group would do.
Fucked Up: look how awesome they look! http://www.youtube.com/watch?feature=player_detailpage&v=mW0-jrDeSgQ#t=101s
D'ya wanna be in my gang?