I think the reasons you do this would be similar to those for why a guy cuts. You may feel you need to be punished. You may consider that harming yourself is a way to feel something, rather than nothing at all. Or you may find that this pain is more bearable because you yourself have caused it; you are in control.
Like I said on that other thread, these are all coping strategies we learn as abused boys. Don't blame yourself for any of this; just get help in dealing with the problem. Any doctor will understand and won't blame you either.
Nobody living can ever stop me
As I go walking my freedom highway.
Nobody living can make me turn back:
This land was made for you and me. (Woody Guthrie)