Like all of the explanations on this site, it's hard to say for certain why any given individual will self injure. There are a great deal of different opinions on the subject. Because of the many different people who suffer from SI, no one explanation can encompass all the cases. I will try my best to give a general idea of what may cause it.

People can do this because they feel confused, or betrayed, or without answers. Feeling frustrated can make you want to do this. A feeling of alienation, a feeling of seperation or abandonment may also cause it. A feeling of helplessness at your situation or one of general helplessness in life can also cause it. Anger at someone who you know you can not take your aggression out on may cause someone to self injure.

After time, the original reason may fade. The self injury may become habitual or ritual. When injured, the body can release endorphines to cope with pain. This is natural. It gives a kind of rush and calm, one which the mind can grow addicted to. If the original reason has faded but self injury continues, the sufferer may find themselves compelled via a craving for that sensation. It is the same kind of "addicition" which people who absolutely love to get tattoos can wind up with. The only difference is society is much more lenient towards tattoos than it is self injury.

Sometimes it is a symptom of a larger mental problem. This is not always the case. However, the DSM-IV lists self injury as a symptom of many different disorders.