Josué's main reason for leaving is because of the gang violence that he has experienced, and he wants a better life for himself. This is another case where he didn't really have a choice.
He had an experience with the Chicos where they asked, "When are you going to join the gang? You'll have money, women, guns, drugs, whatever you want!" and then he tells us, "Something in their eyes told me that if I wanted to be a good person, I shouldn't go with them." He just wants a better and successful life.