Fear is born of natural instincts, instinct for survival, etc. I am not fond of spiders, snakes, etc., but that fear has not progressed to the irrational stage where it causes me grief and harms others.
Fear becomes an irrational belief when it does damage, IMO. I could be wrong, but all human beings seem to fear being alone at some level. That is natural, but look where it leads. Out of that fear are born these beliefs: I'm a loser, when they find out what a failure I am they will abandon me, money=power=people following me, I'll be miserable, I could go on ad infinitum. Those beliefs lead to me playing the bigshot or kissing people's ass so that they'll like me. What happens is that I drive them away and then I end up with I feared to begin with. It is like fear is a self-fulfilling prophesy.