I hear that one reason of cutting, is to focus all your emotional pain into one place, physically, so, feeling the pain, is a way of releasing emotional pain.
It becomes a kind of comfort. Following, it can become an addiction. The body responds, physiologically, in a certain way and this (release of chemicals or the way it makes you feel in your mind) can become addictive.
That is why I have little patience with people who say it is just done by young people to get attention ☝ (Undoubtedly this is in some cases, but that would pass after a while... and that attitude is no help to people suffering from the real problem.) It is *not* just for that reason.
I knew someone who used to put needles into her arms. She said, the feeling of the blood was a comforting feeling... warm... of course, the body can block out feelings of bad pain, so in fact you don't feel the pain (or not badly), this is part of the symptoms. (She had suffered a lot, of many different things... it was to her a way of releasing pain.) Personally, I have done a very few times, when feeling really bad, but this was many years ago now. Fortunately I didn't fall into addiction... it was just pain and desperation, and I realised my behaviour was out of order.
If you know masamania website, there is an article 'crazy wrist-cut girl' which is interesting, if you want to find out...