Every dog goes through two fear-imprint stages: the first occurs when the dog is around eight weeks old, and the second fear-imprint stage occurs at about fourteen weeks of age. It is during this time period that a puppy is more likely to be afraid of the world around him. New sights, sounds, experiences, and environments seem to startle dogs the most during this time.
If, during the formative years of his life, a dog has a very frightening experience that is handled improperly by its owner (i.e., if the owner fails to teach his dog that the object of his fear will not cause him harm and that there is nothing to be afraid of) he may always be fearful of what caused him to become afraid, and anything similar to it. For example, let’s suppose that your puppy was frightened by a friend of yours, who is also a police officer, that showed up at your door unexpectedly (in uniform). If, after feeling afraid, your dog never becomes used to this person, he may develop a permanent fear of anybody who looks like that police officer (men in uniform, other cops, etc).
Because of their breeding, some canines are just more nervous than other dogs, and therefore more likely to become anxious. Some breeds of dogs, usually those that have a higher level of intelligence and those that are emotionally dependent on the interactions that they have with people, are more prone to developing phobias and are very timid. Here are some of the dog breeds that can be classified as anxious: Weimaraners, Great Danes, and Border Collies.
If a dog has experienced a severe trauma or been abused in his past, he may engage in fear-biting. Several dogs that have been abandoned or abused are prone to anxiety, which, if left untreated, may result in fear-biting.