"it's a method of causing mental satisfaction that physically causes bodily harm" Yes but what about if the bodily harm is barely noticeable and very trivial, while satisfaction is more noticeable ? More benefit than drawback, some will say.

I just compare this practice to extreme sports: why do people do that ? Because they like 'hardcore' - it gets them fired up. They take risks, they may die or get their skull pierced and go full-Michael-Schumacher. It's arguable an objectively bad practice. What they do get though is adrenaline. And they crave it so they go for more risk and more hardcore.
Why do some people do self-harm ? To generate encephaline. The organism produces a handful of narcotics that activate 'mu-receptors'. Morphine and heroin are other examples of mu-receptor activators. So you bet for some people (there's always variability of response) the feelings are crazy positive and hard to resist. But as with extreme sports, it can be done in a way where risk is reduced. One doesn't have to go full-yolo about it. There are moderate methods.

So why look down on moderate self-harm if one doesn't look down on skydiving ?