You’re walking down a corridor, you hear somebody behind you. You’re not in a rush, so stand and hold the door open for them, and possibly for others behind them.
They just walk past, not even batting an eyelid at you, and don’t say thank you.
I don’t stand there for my entertainment, I do it as I’m a nice guy, trying to be polite, and trying to make your day easier.
Christ, proper grinds my gears.