It generates endORPHINS, which, like opiates such as heroin (diacetyl mORPHINe), make you feel better, but are just as addictive, and you develop tolerance to them, needing to cut more, and deeper, later on, just to get the same effect. Later in life, most self harmers realise that it is not an effective solution to their problems, and then they are faced with a mass of unsightly scar tissue, which they find difficult to explain, and which can hold them back in their social lives, and careers.
Seek therapy, and learn to address the causes of your problems, rather than coping with the symptoms. View the extensive information on self harm, in section 16, at http://www.ezy-build.net.nz/~shaneris and learn some alternatives, and contact the hotlines, chatsites, or forums, when you feel the need to harm yourself.