Why do people join gangs?

I spent almost 15 years working as a teacher in a detention facility for juvenile delinquents. You won't like my answer. From my own observation, the most common reason is simple: Their father, grandfather, uncles, brothers and cousins are all in the gang. Their sisters are all married to gang members. Everyone in their neighborhood is either part of the gang or admires or at least respects the gang. In most cases, they have known nothing else. In a way, and as they see it, they have no other choice. Television is probably the most culturally enriching thing in their world, and it is probably good for most of them because it shows them something of what the regular world is like.