When I self harm, it's because I can't handle the feeling of the extreme emotional anger and frustration/dissapointment in myself that I can feel swirling in my head, so I would much rather have the physical pain take away what I'm currently feeling. In my case, I will: repeatedly slap myself in the face and/or bang my palm into my forehead many times with the irrational thought of being able to somehow beat my brain into "working better". Kind of like a reverse concussion I guess? More often than that, I will usually punch myself in the thigh excessively to the point of swelling and bruising.
Everyone has their own set of reasons for choosing self harm, but I would like to believe that those of us who are on this path do it for the purpose of not being able to properly deal with the extreme discomfort that is brought on by whatever negative emotion is currently being felt. Or not. Who's to say?