Holding a door for someone who is a little too far away is a kindness that ends up costing them. You see them coming and wait, and now they have seen you waiting, so they break into the apologetic half-jog that nobody does anywhere else, and arrive a little out of breath to thank you for a door they could have opened themselves in two seconds. Once it has started, neither of you can stop it. Somewhere there is an exact distance at which a courtesy turns into a summons, and nobody knows where it is, least of all the person holding the handle.