And what is the best way to treat it? Is it best just to leave the subject alone? It seems like bringing it up just makes them want to do it more.
Probably not a good idea to divulge so much personal information, but I was briefly a "cutter" during my early teens (14-15 ish). It didn't last long, maybe a few months. I just grew out of it or decided it was stupid.
Why? I honestly can't say why. Some kind of fascination, a warped sense of excitement or risk? I have no idea.
To be perfectly honest, I do not see all that much difference - philosophically or otherwise - between the various methods of self-mutilation including the 'accepted' forms. Excessive body piercing and tattooing is just 'prettier' than using a razor blade, but that is about the only difference. Many psychologists have made the parallel.
So when you think of 'cutters', it really shouldn't strike you as so dramatically different than those who run around with a dozen rings and studs hanging from every orifice and protrusion of skin, nor those who plaster themselves with tattoos. If you're not gravely concerned about the psychological well-being of a buddy who is working on his 10th tattoo or scrotum ring, you shouldn't be all that much more concerned about the psychological well-being of a cutter. Same diff.
I certainly do not want to suggest that 'cutting' is just a phase that will pass on its own in all cases. Some will pass, some will not. There are probably a few different underlying psychological factors that cause different people to self-mutilate for different reasons, including those who tattoo and pierce. Some are largely benign, some are very harmful. Some of those factors may resolve on their own, some may persist, or become manifestly worse. You cannot predict or know until it plays out.
Since mine appeared to be a relatively short-lived phase that resolved on its own, I really can't say what the appropriate response should be. Sorry I couldn't answer your question directly.