I have friends who will hold the door for people behind them, and then my friends will get angry if the person doesn't say "thanks." I will never understand this mentality. I suspect it points to some depth of insanity that lies just below the surface of their personality. If you're going to get upset if someone doesn't say thanks, then just don't hold the door for anyone. It's not like humanity got together and agreed on door holding and thanking etiquette. I personally don't like it when people hold the door open for me anyway. They always feel compelled to do it when I have my bike in tow, and then they stand right in the doorway and I can't get through anyway. But then I always thank them, just to avoid the crazy backlash. It's a vicious cycle.