Fear and phobia are often used interchangeably, but they are two very different things. Fear is an emotional response to a perceived threat that does not interfere with one's quality of life. The response to the perceived threat is not so intense that the dog is unable to function. On the other hand, phobias are more intense and impact quality of life. The dog experiencing it does everything he/she can to avoid the object or situation that causes her fear. The dog may behave as if her life is in danger. For example, a dog may do everything in his power to avoid public parks by the beach because he twice heard cars backfire when he was visiting there. A dog may begin aggressively guarding one of his paws from any handling, even though it is not hurting, because a groomer once cut his nails painfully short while roughly restraining him. For some dogs, this would have stopped after experiencing fear during the specific event and it merely would have been a bad experience. For other dogs, it develops into a full-blown phobia of anything that reminds them of the situation.
How does this start?
Some dogs, like people, are genetically pre-disposed toward suffering from anxiety disorders. This can run in particular breeds, but look at the parents of the dog suffering the phobia, if possible. Like people, dogs that suffer from physical pain are more likely to experience anxiety. Often times, if a dog is anxious about one thing, such as noises, you may see other anxieties surface, like dog reactivity.
When puppies are little, people often do one of two things. They either keep them inside to shelter them from the world that might hurt them, or they take their puppies everywhere to expose them to sounds, surfaces, people, and noises. The outside world is a worry for some people if their puppy has not received all the rounds of core vaccines. This is a common scenario despite the American Veterinary Society of Animal Behavior's position that the risk of not properly socializing puppies outweighs the risk of them potentially contracting illness. AVSAB states that puppies can begin socializing in classes at 7-8 weeks, as long as it is at least one week after the first round of vaccines and de-worming. Exposing puppies to the outside world during the primary socialization period can help build confidence while preventing fears from forming. These puppies have a wide array of new experiences during their first three months of life, when they are most impressionable.
Fear periods are also a culprit. I have worked with many dogs that had bad experiences during their second socialization period (~6-18 months), during which fear periods can come and ago. Dogs are more sensitive during these fear periods and bad experiences can imprint for the rest of their lives. People frequently give their dogs more freedom during this period since they have gone through some basic training. They let them off leash sometimes, or take them to dog parks. The scenario I hear the most is that someone's adolescent puppy was playing with another dog at a friend's home, and the adolescent was bullied aggressively by this familiar dog. After this, all dogs were a life or death threat to the adolescent. When on walks, the dog would drool, pant rapidly, and display severe aggression toward other dogs while restrained on leash.
I also see other scenarios involving touch. An adolescent puppy may be handled roughly once by visitors to the home, and now, the puppy growls and bites anyone who tries to pet her. Sometimes, this phobia of touch increases to the point where the dog charges at visitors just for being in the room, and sometimes even begins biting the pet parent for invading her space.
A fear can usually be worked through. A dog that is fearful of a new harness can be rewarded for sniffing and exploring it (Play the Look At That game!), until he is rewarded while actually putting it on. Phobias are not as quick and easy and usually involve a combination of behavior modification and behavior medicine approaches. Desensitization and counter-conditioning are used to address both fears and phobias. Working through phobias requires patience and persistence so that the dog and pet parent can hopefully have an end result of a better quality of life.