I've been hurting myself in various ways for over 25 years. It is like a drug, in the sense that it helps you feel better, but then the more you do it, the worse the injuries need to be to make you feel better again. I've gone from the odd scratch, to having a body mis-shapen from years of deep lacerations and now 3 degree burns. I used to hurt myself multiple times a day and while it's not so regular anymore, the severity is pretty bad.
For me, it's about being able to 'see' the pain in my head. The worse the pain in my head, the worse the injury needs to be. Problem is, I'm well aware my self harm is dangerous, nearing on very nearly could be lethal at times. It's like if I can 'see' the pain in my head in a physical form, it makes sense that I'm in pain. I'm now in a different form of pain which is more manageable than the mental agony I am in. However, it's nothing more than a mere distraction for a while.
Another reason I might do it, is because I sometimes feel like I deserve all I get because I don't matter anyway. Sometimes I do it because I want the choice over what happens to my body (that stems from SA/r*pe and not feeling like my body is my own).