Why do phobias evolve and exist?

I know some phobias can evolve due to post-traumatic stress or maybe the person with the phobia developed it due to having to endure other people's phobias (for instance, my mother once worked with a woman who was severely brontophobic (fear of thunder and lightening). It was because when she was growing up, her mother would go absolutely insane every time there was a thunderstorm). Or sometimes it can come from seeing or hearing about things that happen to other people if it leaves enough of an emotional impact. If one's phobia isn't related to one of those things, I am not sure it is known why exactly the phobia exists. Some people seem to be just born with it or it just comes up all of a sudden. Maybe part of it has to do with hormones but I am not sure...

Phobias are a defense mechanism. They may be triggered by something that happened previously in life, something that affects you directly to this day -- or overall, an experience/trigger that caused you psychological turmoil directly related with that phobia. I honestly don't know of any phobias that I might have. Granted as with anyone, there are certain things that make me a little uneasy, but nothing extremist at all.

When I was a kid, our housekeeper told me about a time when a mouse ran up her pant leg in the barn and she couldn't get it off of her. I became terrified of rodents after that. I guess it happens from some kind of personal experience or the thought process of a personal experience. I have a terrible phobia which controls my life. I HATE rodents. Mostly rats and mice. I know it is a phobia and there is no foundation for the fear but I just can't help the way I feel. I know when there is a mouse or rat nearby, I can "feel" their presence. I can't go into places if I feel there might be rodents there. I know this untimely fear happened because of her story but I still can't get over it.

I have claustophobia.I cannot stand that closed up feeling. I dont know where it came from or how I got it.

i'm scared of heights n closed places..like tunnels...dont really kn y..n u're scared of right side..?why dextrophobia.?