Hm.. I don't like to hurt people's feelings mostly because, if I do, I worry about it. Especially people I'm close to; or even someone I used to be close to. I worry about it.
And here's why: If I know (or could possibly expect) I did something that REALLY REALLY hurt someone (or failed to meet up to my own standards), then I will recall it at a later date and I WILL replay the situation in my head, and I will punish myself for it again, try as I might not to. I'm not just talking something that happened last week; I often beat myself up for something I said nearly 7 years ago in high school.
I've done some reading on the subject, and apparently it's my Enneagram Type that causes this sort of behavior. 9's feel that when we break the peace, we've done something horrible and upset our own balance. That means that we punish ourselves over it again and again, because we shouldn't have caused the conflict that we created. INFP + 9 = somewhat neurotic about being nice to people and living up to "ethical standards." Though I still mess up occasionally, I've learned to let go a little better now that I'm an RA. You have to. People get mad at you for things all the time, and you just have to grit your teeth and move on.