I don't know if my experience is similar or helpful at all, but I really relate to alot of what you're saying. Alot of times I wonder if it is actually a bad thing because, in my case, it helped me feel better (or feel anything). I've also harmed myself or atleast wanted to strictly due to boredom. Ironically that makes me believe I'm seriously ****ed up and want to SI more, for emotional reasons. Honestly I think this reason may be just as dangerous if not worse than SI-ing because of depression/anxiety/etc.

If it's just something you consider "fun," how did you begin to do this? Was there any emotional issue going on, even lack of feeling anything when it started? There's probably more to it than you realize. Typically the reason for deciding to SI without any known reason is because a person has become addicted to it, but you said you only do this "every once in awhile" so maybe it's not the case. Do you think about it often? Have the urge to and it distracts you from other things? Those might indicate that it's become a habit, which would partially explain what you're asking.

One last thing, you said you do it for the pain which is your goal. Maybe you simply like pain. There is some fancy scientific name for this, I've forgotten it at the moment though. I'm sure there's another fancy scientific word for liking blood too.

No matter what the cause of this is, good luck in figuring it out and, if you ever decide to, good luck in stopping SI as well.