It's bad because you're injuring your body. You can get an infection, you can have severe blood loss - you're hurting your body. After a while it becomes an addiction, an addictive behaviors are hard to quit. It's a bad coping mechanism. Some people drink or smoke when they're upset, and others cut. I personally see it as a bad coping mechanism, while I've heard others say it's a mental disorder. Whatever your reason for cutting is, you need to stop. Cutting is bad because you're punishing yourself to escape the pain from something else. It's a distraction from whatever emotional pain you might be experiencing. There are so many better ways of coping with whatever issues you may be experiencing. Please stop cutting and try to get help. Please don't kill yourself. Your life is important. There are great people here on the Teen Help forums who will agree with me.
Just talk to us, we'll help you get through it as best we can. Just please don't hurt yourself. Please don't kill yourself.