Sometimes a neighbor you haven't met is the one ringing the doorbell.

I recently had to leave my car parked in front of a stranger's house, so I rang the doorbell to let them know, but they didn't answer even though they were clearly home. Capitalism might be great and all, but door-to-door marketing and phone spamming have killed reasonable methods of communication.

I refuse to let these things die, so I still open the door if I don't know the person (I have a hotel-like door latch now to help prevent someone from kicking in the door or shooting an arrow at me). You just have to learn to be aggressive and say "I'm not interested" if it's a salesman or Jehovah's Witness.