system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do people have irrational fears or phobias?

I mean, my “fear” of flying is irrational because I know all the statistics, I know it’s the safest form of travel and that last year there where less deaths caused by aircraft accidents then there were during a week of car crashes in australia. Yet I can’t get on a plane, the anxiety and fear overtake me so much. Why?

I mean I assume that seeing every episode of air crash investigators might not have helped but I’ve also seen every episode of CSI and Bones but I’m not terrified of being murdered.
gpt
People have "irrational" fears for the same reason they have "rational" fears - the brain thinks that reacting to them will increase your odds of survival in a fight-or-flight situation.  Basically, the reason some fears might seem irrational is that there is a disconnect between conscious thought and your brain's fight or flight system.  This disconnect is what allows you to react to something quickly before you consciously analyze the situation.  

The reason some people have certain fears  (bees, heights, spiders, etc) and some don't is that a person's brain may or may not have a learned or innate reason to fear something.  Innate factors are behind most phobias that don't stem from experience or an event, but to the brain, they carry just as much weight as learned phobias that stem from experiences.