Why is self mutilating addicting?

Coping: People who self- mutilate may do this as a coping mechanism. Physical harm to the body may distract from emotional pain. Some people feel empty inside and hurt themself so that they won't feel completely numb. Although it is a very unhealthy way to cope, people may return to self- mutilation over and over because they don't have more effective coping mechanisms.

Endorphins: Though it is much more serious, the underlying physiology is similar to why people become addicted to to very hot foods: the pain releases compensatory internal opioids (endorphins). These diminish the physical pain somewhat, but of greater importance and concern is that they provide a temporary "high" that alleviates the psychic pain briefly. Its like a quick shot of morphine from within.