I bite myself (either my finger or the side of my hand) when I get excited because otherwise I feel like I'm going to explode.
Also when I'm REALLY anxious and alone outside, but that's not too often.
It's not too bad when I do it in front of people I know, but sometimes I do it in public and I often don't even realise I'm doing it until someone gives me a funny look.
Sometimes I look at my hand and there's fresh bite marks that I have no memory of doing.