Let's see here...
I struggle with si mainly because of the bouts of serious loneliness that happen to me where there isn't really anyone around to talk to or even to make small talk with. I agree with Sam2 that some are trying to stop emotional pain, but for me, I think it's more to get rid of feeling so numb. Feeling pain instead of nothing kind of thing. I started when I was 16/17, so about 3 years. My frequency has gotten worse over time. There was about 7 or 8 months between #1 and #2 and it slowly shrunk from there. It seems to be about every two or three months now. Which I know is nowhere near as frequent as a lot of others. I don't cut or burn, I do something else (which I'm not going to explain because it's unusual and I don't want to give others any ideas or anything) that leaves me with a fairly large scrape. If people notice and ask about it, I make up an excuse. I'm pretty active, so I blame it on a bike accident or falling off a skateboard. My mom thinks I just have a hard time healing. I haven't found the courage to tell people that they're from si yet. I'm relying on myself to quit for now. I'm a pretty independent person, and I don't have anyone that I feel will understand if I tell them about it. I decided to quit shortly after my last one about a month ago, so we'll see in a month or 2 if I need help to prevent it or not.

If I remember correctly, there's a sticky thread on this forum saying things to do instead if you feel like si. So you could check that out for ideas if you need some. Everyone is different, and I think that relying on myself will work for me, but I did read a study somewhere that said if you tell people that you're going to do something, you're more likely to do it.