Holding a door for someone who is a little too far away is a kindness with a small tax on it. They have to hurry so you haven’t stood there for nothing, and you have to keep standing there so they haven’t hurried for nothing. For three or four seconds two strangers are each putting up with something mildly inconvenient purely for the other’s sake, and neither of them asked for any of it. I can’t think of many arrangements that turn so little awkwardness into so much goodwill.