Fear of something can lead to phobia and disrupt normal day-to-day existence of a person. The good thing is it can be treated and be reduced to a level where the person can function normally.
The list of phobias is as long as they are fears. Some fears are learned and some are born out of experience. If something happens to a child, for example, they may develop a fear of the cause of the incident. Or they may learn to fear certain things from hearing comments from parents or other people around them. Some phobias have no logical explanations, and may seem quite unreasonable especially in the case of fear of harmless objects. Some people for example, are afraid to fly. People who afraid of flying, will not make a plane journey no matter how convenient or important the journey is.
Some people are afraid to be alone even for very short times. They imagine that the worst will happen to them when alone. Phobias are thought to be a kind of mental illness. They will be very afraid of venturing out on their own.