How come teens get addicted to self-mutilation?

Endorphins. Self-mutilation is often a way to feel better and deal with painful thoughts, feelings, or memories. Self-mutilation likely releases endorphins in the brain. These endorphins feel good and stimulate the pleasure-reward pathway, which reinforces the continuing self-mutilation. Some clinicians prescribe naltrexone, which blocks the good feelings from endorphins and may help break the cycle.
Personality. There are problems ranging from body image to borderline personalities who only feel something when the do this.
See below. Self injurious behavior (sib) is clinically seen in the context of borderline personality disorder in the case of adults. It is however more common in the teen years and occurs when emotional distress meets poor coping skills. Self injury numbs the emotional pain in teens, who have a natural imbalance between the feeling and thinking parts of their brains.