Well, actually, you're not
alone. I used to do that a lot, until people around me noticed the
wounds and started to get
concerned.
I don't know about you, or anyone else, but I did it to get even with myself for the horrible things I've done. There are two of me a lot of the time, it seems (there must needs be opposition in all things, right?) -- one wants us to survive and the other wants to destroy. Not just the body, but everything we've ever worked for. Drives me to do things I know are dumb, things I know I'll regret, just for the sheer pleasure of destruction.
Destruction gives euphoria. Even when you do something purely physical, like slice into your skin with a razor blade, your body promptly doses you with natural opiates. I think you can become sensitized to this, and start getting a rush from it. This is why, even though I may have promised the others that I won't cut myself anymore, I "accidentally" burn my hands on hot objects or bite my tongue until it bleeds (this last typically occurs when I am struggling to resist blurting out hurtful comments to fools who insist on being in my presence (don't lie to me -- I know you too have "bit your tongue" at times)).
We destroy ourselves, some of us (I can't speak for you, but I can speak for me, and for every me there are an undetermined number of people like me), in bits and pieces, because self-destruction is the true power trip.