Why are people drawn to gangs?

What is it that draws people to gangs even though they know that, at least for some, it will include breaking the law in some way at some point?

I ask this as I saw a trailer for a program about biker gangs in the US and indeed it appears to be a part of a series about them and I am interested in the psychology that lies behind it all. What triggers it in some people, but not in others? Have you ever been in one and if so what led you to it?