People sometimes do this because when they're sad or depressed they may feel powerless and by hurting himself in some way I don't really understand though it makes them feel like they have power over something. The best thing to do is tell a trusted adult what's going on so they help you or someone else get the help needed.
People cut themselves when they are mad or upset as a way of relieving the pain they are feeling. They also do it because they like the pain. It is important to take such a person to the doctor because they might be suffering from Dissociative Identity Disorder and they may need medication.
I don't know. I did it when I was depressed and it helped for maybe an hour but it's worse looking at the scars now. I would rather have gone through the depression for another month then live with these scars for this long. By the way, it's completely weird that I did it and don't recommend it. I'm completely don't with that and I'm never going back. Ever.