Biology and Genetics. Both learning and genetics can play a part in the development of phobias.

When someone develops a phobia, they quickly learn that they feel anxious when they are near the object or situation they fear - and that they feel relief when they avoid it. They learn that avoidance can reduce their anxiety (at least for the moment) and increase the likelihood that they will avoid the feared situation or object next time. The difficulty is that these avoidance behaviors have to keep increasing and happening even sooner to provide the same relief. Pretty soon, a person finds himself spending time worrying about the possibility of encountering the feared situation and avoiding anything that might bring him into contact with it. With a phobia, the pattern of anxiety, avoidance, and worry about the possibility of contact tends to grow bigger and interferes more with life over time.