Many people join gangs because of the environment they live in. Whether it's a person's home life, or life in the community around them, people are influenced by their surroundings. Look at it like this, a boy around 13 years old grows up in the P.J's, and all he sees is slangin' and gang bangin'. His family is poor, they don't have any money, and yet he sees people a
little older than
him rockin' the latest gear, and beatin' down the block in the new Chrysler 300 sittin' on some 22's. For a kid with nothing and who has never had anything, gang bangin' might be the only way out that he sees for himself. Not all people who join gangs live in the projects and in ghettos, some of them have it really nice, but are influenced by other things, such as the music and movies they hear and see that glories the life, while others are influenced by family member who have accepted the life. Young people are more easily influenced than adults, so it becomes very important to give the youth a positive environment, so that they might have a greater chance for a more positive future.
Many people see it as tradition to join gangs. Growing up, a person might have a father or a mother who was apart of a certain gang, a few siblings who belong to that same gang, so naturally when thy are able to make that decision, they'll choose to do
what their family is doing. Or maybe it's a community affair, where that whole hood or area are a port of the same set, and it's tradition for the children, when they grow older, to represent that set.
Some kids growing up may feel the need to be accepted and wanted, and they see how gangs are close and work as a unity, so they might be inclined to join gangs. 
With that acceptance they find, they also find respect, money, friends, safety, and lust; it's kind of like another family to return to when in need, and in that, they find comfort. Some kids might get pressured into gangs, whether it is “positive” or negative. Some “positive” Peer pressure might be your friend that always has money, cars, clothes, respect and popularity, you want in, so you do the do and jump on the bandwagon. Others might get forced into it.
Some are threatened their lives, or their families lives, while others, feel the pressure of keeping up family tradition. Whatever the pressure may be, not all gang members are there because they want to be