Phobias are common and lead to important impacts on many aspects of your life

It is an intense fear with all its bodily experiences, 

Causes

Learning Theory - Pavlov - says that phobias develop when fear responses are reinforced or punished. Both reinforcement and punishment can be positive or negative.
A phobia may have been learnt, e.g. you saw someone screaming and you learn this was a very dangerous situation to be in;
or you directly experienced the connection fear-life event:
It can follow a traumatic event

Psycho-analytical theory - Freud - states that the currently feared object is not the original object of the fear

Biological theory: 

mental disorders are caused by physiological factors. This theory focuses on neuropsychology, which is a branch of psychology that is dedicated to studying the structure and function of the brain.

Neuropsychologists have identified certain genetic factors that may play a role in the development of phobias.


 Reality is that the situation is often multi-factorial and best is to listen to your story with your specific phobia