People who are sadists (Those who inflict pain on themselves), have an urge to cut themselves to release the pain inside. My teacher told me that is gives out a hormones that soothes the user's feeling.
Its very stupid to cut yourself, as you are only hurting yourself and no one else. Its best to either kill yourself or not hurt yourself at all. When i was young, i beat myself in the head with a hammer. Stupid as fudge