SI is an addiction. It causes the brain to release endorphins which make you feel good. Over time, we become addicted to that chemical and seek it out. Endorphins are also released in the brain when you eat chocolate, spicy food, exercise, have sex, etc.

The act itself also provides us with a way to have some control in our lives and to also express our internal feelings.

I'm not saying this will help you out when you're struggling, but it does answer the why.

Try to be patient with yourself. No matter what the type of SI or the type of addiction, there will be ups and downs. One thing I try really hard not to do is count the days I've gone w/o SI. The higher the number of days w/o it, the more pressure it puts on me.