Thank you?

When somebody does something nice for you such as opening a door, the polite thing to do is to say thank you. The person is stopping for a second to prevent a door from slamming into your face and so a few words of gratitude, are required to show the person your appreciation. However, if someone is holding open an automatically opening door is this really needed? Yes they are putting in the effort to be nice, but the door is going to stay open regardless of what they do. You don't really want to make them feel good for what they are doing, because all they really are accomplishing is making themselves look dumb for holding open a door that isn't going to close anyhow. But if someone holds a door open for you, the right thing to do is to say thankyou. Such a moral delema...