heebie jeebies

i think most of us are familiar with the tv show monk.  i caught a rerun the other day and i had somehow forgotten just how many things he was afraid of.  every episode he had to solve the cases while somehow dealing with his seemingly infinite amount of irrational fears and phobias.  phobias are an interesting subject to me.  obviously they are caused by situations that could possibly cause you harm, but i don't understand why some people are afraid of some things and others aren't.  thinking about my fears, i am afraid of heights, spiders, open water, falling down stairs, small spaces, being in my house or apartment by myself, most large animals, people in masks, rodents, getting stuck in an elevator...i could probably go on for days, so i'll just stop there.  but my roommate has no problem with heights, open water, stairs, small spaces, being alone in the apartment, people in masks; in fact, she probably doesn't even have half of the same fears i do, but she definitely has fears of her own, some of which i might not have.  also, unlike my sister, who is an ever bigger scaredy-cat than i am, i am not afraid of ants and lady bugs, trust me, i know she's ridiculous.  what i don't understand is when you are dealing with things that can cause you harm, why isn't everyone afraid of it? why aren't we all afraid of the same things when we all have the same chance of being harmed?