Because teenagers have not yet fully developed the parts of their brain responsible for impulse control. Add into that mess heightened hormone levels screaming

Because teenagers have not yet fully developed the parts of their brain responsible for impulse control. Add into that mess heightened hormone levels screaming to fuck or fight, and you've got a recipe for misbehavior. That can be overcome with good parenting, but you don't have to get a license to pop out a brood.