For me, it was a choice between THIS MUCH emotional pain, or this much physical pain. Given a choice between an evening of misery and self-loathing or a self-inflicted paper cut, the paper cut was an easy choice.
The reasons that physical pain can substitute for emotional pain get complicated, and I can only speak to my own experience here, but I’ll take a shot at it.
All pain triggers the body to release endorphins, but damage to the top layer of the skin releases endorphins like crazy. This is why many people who harm themselves stick to shallow scratches, it’s got a better chemical payoff than other forms. If the endorphins put an end to the current spiral of mental suffering, mission accomplished. It’s a bit like going for a run when you’re upset, but faster acting and way less healthy. It takes more every time, though, and for many people it becomes an addiction and takes on a life of its own. YMMV, but that’s usually how people end up cutting so deep they end up in the hospital - they didn’t get there overnight, they’ve been working up to it for a long time. They really didn’t intend to damage themselves so badly, they were just trying to get their fix and it got away from them.
When my therapist pointed out that self harm was an addiction it took all the fun right out of it for me. She was right, I was doing more damage every time to get the same results, and I was starting to cut for the sake of cutting rather than to deal with an immediate emotional crisis. Addictions run in my family and I’ve seen what they can do, so I immediately cut back (no pun intended) on how much I was doing. I stopped for good six months later. I was lucky - I was a very mild case, and I got help early. I was actively cutting for maybe a year. I still thought about wanting to cut every day for five years after I stopped.
The endorphin rush is one of the payoffs of self harm, but there are others. It can also be a form of self-expression. One therapist told me that it’s common in young women (and presumably young men) who have been objectified: “Look, I’m real. I can bleed. I’m not a doll or automaton.” It was also a way of proving my own toughness in a twisted sort of sense: “You all think I’m weak and worthless, but look at this. I can handle more suffering than you know exists, and I can heal it. I am strong.” (Watching cuts heal was the best part of the whole thing for me.)
As others have mentioned, it can also be a form of self-punishment. My first foray into self harm was after having non-arguments with my dad as a preteen. He was extremely self-centered and controlling, we were not allowed to be mad at him. If we felt hurt or upset about what he did then that was our own fault. I had a weird sense of the world being out of balance - something was very wrong here. Of course it was my dad’s behavior that was wrong, but I wasn’t allowed to express that. Somehow punishing myself (by beating on my legs with my fists) balanced the scales again and my world was no longer in danger of exploding.
So for a bunch of reasons, self-harm can be incredibly cathartic. It’s a way of self-medicating emotional problems, but it can become a problem in its own right and (IMHO) needs some specific attention for recovery. Recovery is possible, though. There’s a better way to live. Really. I had no idea that life could be good until I took the plunge to try and heal. I’ve been basically happy for the last few years, and it still astonishes me.
My apologies for the novel, I hope it’s of some use to those who are reading it. Inigo, the best thing you can do for your daughters is just to be a loving parent. It’s okay that you don’t entirely get it, my Mom never did either. It was actually a good reminder to me that healthy people don’t think this way. Best wishes to you and yours. Self harm is a real bugger, but it can be treated.