These are my thoughts.
A lot of people who want to commit suicide don't do it because they're scared. But once you start hurting yourself, it gets easier to do it again. After a while it becomes an addiction. Just like drugs or alcohol, it becomes a fail-safe way of numbing the emotional pain, but you have to get it more often as you become less able to deal with things you could before. The more you hurt yourself, the further you're willing to go, until eventually you want a more permanent fix and do what you couldn't have done before: Suicide.

I'm not saying all self-harmers are suicidal, but I'm pretty sure it increases the chances of being that way. If I'm wrong about any of that, feel free to tell me.

Sorry if it seemed clinical or lecturous.