I do it too. We talked about this in group therapy, but I can't remember what was said exactly. But it did go along the lines if self esteem. I know when I was a kid it was pretty much taught to us by our parents. Hard thing to stop doing but I'm trying to stop. For me I know I feel guilt over even tiny things which drives the appologies. Growing up my parents used a lot of guilt tactics on us. So I think it can be a learnt behavior as well as a self esteem issue.