I feel like Wild Coyote is on the right track with her belief that this is often emotionally driven. I have perhaps a slightly different take.

I feel like our emotional wounds are often hidden from the conscious mind, but they are front and center for our subconscious. It is often very difficult for us to believe in or prioritize taking action against what we cannot see. I feel like then the subconscious mind then leads us into activities that harm our physical bodies. Hence we get the 'I cannot control it and I don't know why I'm doing it effect' because the subconscious is not rational.

Perhaps self harm is the mind's way of literally showing us that we are in pain and we need to take action. The physical manifestation of the harm then serves as a metaphor for the emotional pain.

Perhaps if we looked at physical harm for meaning and connection the way we do dreams we might be able to decode it a bit and address the root problem that is manifesting in physical ways.

Just a theory...