To punish myself. That was the reason the very first time... when I was... like 12, I think.
Now it varies, generally it's still to punish myself.
There are times though that it's easier to redirect the pain and have it on the outside and controlled instead of inside.
Most of the time, it feels like I'm more in control
Every once in a while it's felt like I had no control, and those are when the scariest incidents happen.