I have no idea why some people believe it's rude not to answer your own door. Okay, I do answer the door the TV Licensing goons, but only to slam it in their face. But for other people, I sometimes don't bother. After all, it's my house and I do not have to answer my front door. I don't need a reason.
So if - let's choose randomly - an elf collecting money comes to my house and rings my doorbell, I might not bother with the door. I might even walk past the window so the person can see that I am in. The person becomes annoyed or frustrated at this lapse in social etiquette: "Why didn't you open the door?" It's simple: it's for whatever reason I can be bothered to choose. Maybe I'm asleep. Maybe I'm taking a shower. Maybe I'm having a crap. Or maybe I'm blatantly and openly ignoring you as I walk past the window. Maybe on your bike. After all, it's my house. You took your shot to see if I'd answer and I didn't. Move on.
