Whether a dog is very small with a yappy bark or a big dog with a deep throated growl, they all think of themselves as fellow wolves in a pack and the home where they live as their personal territory. Small dogs can have strong personalities for this reason. The mailman is a stranger and they are seen as a member of another pack which demands an immediate challenge. Of course, the mailman quickly delivers the post and then exits as fast as he or she can. The dog feels that he has been successful. This is a version of reward based self-training. We know how you should train cats and dogs. You do it through positive reinforcement. When a dog barks at a mailman with apparent success it is a form of positive reinforcement so they’ll do it again.
Then the mailman returns the following day and the dog decides that he has to bark a little louder and be a little bit more aggressive in order to achieve the same result. This is why dogs try and attack the mailman. Fortunately most of the barking and aggression is behind a closed door.
The dog clearly considers the whole of the front yard from the gate of the front yard to the front door as his territory. This may be a legacy of hunters leaving discarded food around the encampment where they lived. The food belonged to the dog and it lay in an extended area beyond the boundaries of the building. We all know by now that domestic dogs are pack animals by nature. They have a leader, the dominant dog who is the man or woman or couple who adopted the dog. The top dog is a human and their dominance is beyond dispute most of the time.
It is said that dogs are pack animals because they work together to own a source of food. And if food is scarce they need to work together. But if these circumstances are not present they may not form a group. Although dogs need a pack at least of two individuals. It provides them with a sense of security and companionship. It reduces anxiety and allows them to grow in character. Dogs need to have a sense of belonging to a family unit.