You are on the right track with figuring out why. We do everything for a reason. When you know the reason you can find another way to get your needs met.

The reasons are different for each person though, and often not even the same for one person every time. What are you thinking or feeling before and after you SI? What changes? How do other people react when you do it? Answering these questions can help you figure it out. If that doesn't get you there, write in a journal and every time keep track of what was going on before, what your thoughts and feelings and actions were, what happened after, and why you think you might have done what you did that time. After a little while, you can start to see patterns.