I have been self-injuring (cutting) for six years. I have gotten help several times, but I always just relapsed again. I do it because I feel like I'm worthless, and stupid, fat, and ugly, and that I don't belong here. I've tried to kill myself several times. I have been abused and molested by my brother, and I have Severe Clinical Depression, Bipolar DIsorder, ADHD, OCD, and three different anxiety disorders. I am medicated for all of them. No offense to anybody, but I find that adults, and older people don't understand self-injury. They say it's just a "fad" and everyone will get over it. Usually, the person isn't doing it to be cool. They're doing it because they're NOT cool. They're depressed, and hate themselves. I'm currently in therapy, and it's helping a bit, but it's a hard thing to deal with, and it's almost an addiction. When you get injured, the brain releases endorphins, giving you a natural high. These endorphins are released when you cut yourself, as well.
I hope I helped you understand self-injury a little bit better, and different people do it for different reasons. If you know someone who is cutting or harming themselves, I suggest that you get help for them right away.