This one haunts me all the time. I actually LOVE doing things for others without being asked. I love surprising people with little things here and there, it makes me happy to see their reactions... I don't think I care if the favor is returned, when I do things randomly it's almost embarrassing when they feel they have to do something in return.
HOWEVER... there is nothing I hate more than getting taken advantage of. I hate it when I try to do something nice for someone only to find them expecting it of me. I lose the ability to do things just to be nice. It makes me seriously question a friendship when I am the one being put out. So, that said, maybe I do think about it a little. I LOVE my friends who are GRATEFUL of the things I might do. I WANT to do more for them when they thank me. It's not that I expect gratitude, it's just more fun to do things for others when they are grateful for it.
Maybe life would change for me if I just let myself get taken advantage of, but I don't think it would change for the better. I think it's OK to pick and choose your friends carefully- cause a friendship can get tired quickly if one person is sapped dry!