It feels complicated for me. Sometimes I end up doing it just because I feel like my mind has bombarded me with the option until that's the only way I can get relief from that. That makes me think that there are a lot of reasons for the behavior but that they're not conscious reasons.

So far I've come to understand that part of it is to punish myself, part of it is to relieve anger without breaking things that do or don't belong to me, and that part of it is the enjoyment of having scratches and scars like I did as a kid from just rough-housing.