Fear is defined by Wikipedia as an emotion induced by a threat perceived by living entities, which causes a change in brain and organ function and ultimately a change in behavior, such as running away, hiding or freezing from traumatic events. Fear may occur in response to a specific stimulus happening in the present, or to a future situation, which is perceived as risk to health or life, status, power, security, or in the case of humans wealth or anything held valuable. The fear response arises from the perception of danger leading to confrontation with or escape from/avoiding the threat (also known as the fight-or-flight response), which in extreme cases of fear (horror and terror) can be a freeze response or paralysis.
Animals experience fear in the same ways that humans experience fear. For example my youngest daughter is terrified of spiders. She did not have this fear until she tried to pet a fake spider on Halloween night and it jumped out at her. Now all spiders real or fake send her into a panic. She has generalized the fear to all spiders both real and fake. Our animals do the same things however just like humans they do not even have to have had a bad experience with the object of fear, all they need is to perceive it is a danger to them and they respond with fear.
When I first got Yukon he was terrified of leashes, people, and coming inside the house. Tashi was terrified of metal, metal stairways, metal gates, cage doors, etc. I am working with another dog who is just a pup and has nad no negative experiences with people however he has started to growl at them and he sees them as a danger. Since a psychotherapy session would prove ineffective for all of these dogs we have no way of knowing whether they had a bad experience with these things or if they are generalizing from an experience to something similar. The facts is that these fears are real for these dogs so the only way to respond to these fears then is with compassion and positive training. The old protocol was to flood people and animals with what scared them most. The studies found that this was ineffective because our bodies then reach a point where we shut down completely or in the case of dogs they often turn on their fight mechanism and now you have a fearful highly aggressive dog that is facing euthanasia.
A fearful dog is delicate so when looking for a trainer be sure that they are not using flooding to treat fear. Look for a trainer that uses positive methods and trains with compassion for your dogs fears because for your dog they are very real, and very scary.