So many reasons. Influence, peer pressure, survival.

They think that they have no other options

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.

For me it was probably a combination of this movie and AC/DC. [youtube=https://www.youtube.com/watch?v=YeEbtiruH2Q]

Safety in numbers ? Just like the tribal days of old ?

Because it's like a replacement family.

For fun or they lack real family

Stupidity

There weak and not got mind of there own , and cowards, think there hard in a crowd but on there own there nothing

They want reassurance from like minded people that they're truly being the best ignorant person they can be.

I didn't choose the streets, the streets chose me.

Also low self - esteem.

Peer pressure is rubbish answer have kids got no minds of there own today

Like violence when in a gang on there own wouldn't say anything

Protection, money, they like violence

Peer pressure &/or stupidity.

A sense of worth

Protection, a sense of community or family in situations where community or family is lacking.

the money

Strength in numbers.