Heredity: Traits such as personality, weight, and mental disorders can be passed down from parent to child.
Upbringing: If a teen grows up with parents who do drugs and believe it to be socially acceptable, they're more likely to do the same. This goes for a lot of things and a lot of behaviors.
Brain: Many teenages are going through puberty and have raging hormones, making them act impulsively.
Explanation:
I'd look into Nature VS Nurture for more information.
