Habits of avoidance are maintained by fear of punishing stimulation. You encounter something that hurts, and thereafter, you don't want to encounter that thing again and start avoiding that painful thing, and even things that remind you of that thing. The pain you feel might be physical in nature, or emotional; it doesn't much matter. What matters is that it hurts and you believe you cannot tolerate it; that it will be easier for you to simply avoid experiencing it again than to try to come to grips with it in some other more direct fashion.
Agoraphobic people provide a beautiful illustration of this principle. Agoraphobia means "fear of the market place" when taken literally. It describes a condition some people have where they become so fearful of going out of their homes that they stop doing so, at least when unaccompanied. Most agoraphobic people suffer from panic attacks, which are frightening and sudden episodes characterized by dramatic physical and emotional symptoms. During a panic attack, people feel like they're having a heart attack, or dying. The experience is terrifying and unpredictable, and so very often, people who have had a panic attack in a particular location come to believe that they can avoid experiencing another attack by avoiding that location in the future. As more attacks occur, they come to avoid more and more locations, until ultimately the only place that feels safe is home.
Sometimes the things that people come to avoid are truly dangerous and deserving of being avoided. Other times, there is no actual danger present, or really only minimal actual danger. In many cases, feelings of anxiety and panic cause people to think they are facing a real and profound danger, when in actuality, they are not. In the case of agoraphobia, the panic attacks feel very dangerous indeed, but they are actually not physically dangerous. The avoidance habit that people get into around their panic attacks is thus out of proportion to the actual threat they pose. You can explain to agoraphobic people that they are overreacting all you like, but this knowledge is unlikely to help them, because there is a natural disconnect between what you know intellectually, and what you know experientially. Panic attacks have to be experienced as not being dangerous before they will stop seeming dangerous to the people who have them.