Agree with complexity and personal safety not being a concern. It can be about control...inflicting our own pain (as weird as that sounds, wish I could explain better). Mine started as self punishment and also need for control...anger and need for power that couldn't be turned outward so got turned inward. Also, I sometimes think my urge to harm myself is similar to a "freeze" response, so like two conflicting impulses. For cutting, it's the urge to fight and the urge to be held tight or be protected...I can't say why, or for sure, but my body seems to need those two opposing things. Probably came from simultaneously wanting to protect myself from caregivers and also be protected by them (not possible, or not a trust-worthy source, so I have a really childish feeling of protecting myself when I'm in crisis, and it involves sleeping with a knife...or even holding it against my own skin). I was hurt most by people who were supposed to love and protect me. In a disconnected place, it's like I'm playing a couple roles...mine and theirs, and by hurting myself I have control over all of the pain (okay, this barely makes sense). I also learned that feelings were not really okay...and strong emotions were controlled through violence or destruction. I'm sure there is some brain chemistry involved here...the whole endorphin release or whatever...cutting did used to help me chill way out.
It might partly relieve dissociation or disconnection from the body. I also went through a phase of hating my legs so badly that I cut them up in elaborate ways to make sure I'd never wear shorts again but hide my legs forever. So, these are a lot of complicated possibilities.
It's hard to find a good substitute. First I had to want to stop punishing myself. If I felt I deserved punishment, it was really hard to find a healthy substitute. Then I had to find ways to release the energy. I don't cut but I think about it often lately...it helps me to imagine myself bleeding, to squeeze my arms, and to squeeze a closed blade in my hand. So lately, feeling protected but also having some visual and tactile way to connect back to my body and also imagine a "release" of sorts. I can't recommend this because it's probably triggering (imagining cuts and bleeding), but when it's really bad, it helps on some level.
It seems like self-harmers generally lack the ability to self-soothe, but also that most average coping skills don't work for people who cut...that's why they cut. The need for comfort is triggering in itself because, for me, it's loaded with risk and even self-loathing. When triggered toward self-harm, my stuffed animal or my pets can't sooth me. I'm not just a little stressed out. I think it's mostly gnarly trauma energy (for me, it seems like fight + need for protection all twisted together), so the squeezing, or finding something to push against, and ways to feel protected...so letting my body turn the anger outward or release that energy safely. Yesterday I wanted to cut and could not turn any of it outward but realized I had a real need for protection. So I've just imagined crawling into a shell, avoiding other humans, trying to manage pain and triggering stuff.