It isn’t hard. It feels hard! When you walk through a door right before someone else, you need to hold that door open for them, or else you’re rude. But if they’re a little too far behind you, they have to hurry to catch up, and then (as Redditor Voldetitty recently pointed out) you’re actually being annoying. At some nebulous distance there is a phase change from “don’t hold the door open” to “hold the door open”, and misjudging it will ruin your life for as much as five seconds.