I read something once that basically said we hold doors open to be polite, yes, but we also want to be commended for it; that's the thank you part. When someone doesn't say thank you we are offended because we have not been recognized for doing something nice. I'm not saying this is an absolute truth but there is some validity to it. Then of course there is the whole common courtesy aspect of it and the idea that simple things like "please" "thank you" and "excuse me" have gone the way of the caveman. Honestly, I have pretty much stopped holding doors open for people here at work because no one ever says "thank you."