There is never a simple answer to that question. Some people use violence to release feelings of anger or frustration. They think there are no answers to their problems and turn to violence to express their out of control emotions. Violence is used as a way to control others or get something they want. Violence is used to retaliate against those who have hurt them or someone they care about.
Violence is a learned behavior.
Like all learned behaviors, it can be changed. This isn't easy, though. Since there is no single cause of violence, there is no one simple solution. The best you can do is learn to recognize the warning signs of violence and to get help when you see them in your friends or yourself.