Imagine the following scenario: you live in a world full of threats. You are in charge of guarding your home against creatures about five times as big as you. One day, a huge creature starts messing around with your door. You yell threats: "Go away! I'm warning you! Don't you dare try to get in!" And after a minute of your threats, the creature leaves. Then the next day same creature comes again, and you once again drive it off. Every day this creature comes poking around. Think of the fear, the feeling of responsibilty, the stress.
This is exactly how the dog feels about the mailman.