Dogs protect their owners because of the compound results of training and instinct. Dogs are, by instinct, pack animals that thrive in a social hierarchy. The dog owner is the pack leader, and any perceived or actual threats to the owner are addressed through protective behaviors by the dog.
As pack animals, dogs typically assign a higher or lower status to others in their pack, including the owner. A well-trained dog is submissive to the owner and is ready to defend the pack leader in the presence of perceived or actual threats. Also, since the owner is seen as the dog's provider of food, water, shelter and attention, it is in the dog's best interest to act in defense of its livelihood.
Although protective behaviors range across dog breeds, they can include barking, howling, growling, lunging, snapping and, ultimately, biting. Dogs typically demonstrate different combinations of body postures and vocalizations to communicate warnings of attack to would-be challengers. A major exception here is the chow chow, a breed of dog that may give little to no indication of aggression prior to biting.
Early introduction of the dog to training and socialization with humans and other animals can help to prevent the frequency and occurrence of unwanted protective behaviors.