It has an evolutionary history that helped animals adapt to survive in their environment.
Fear of reptiles may have dated back to earlier times when mammals tried to avoid being eaten by dinosaurs. With that being said, I would like to state that I have a monumental fear of snakes.
I don't recall when the fear came upon me, or if an experience prepared me to become afraid. But the thought or vision of snakes give me the heebie geebies. The book mentioned several experiments with regard to fear. The model Monkey and the observer Monkey, the phobic stimuli versus the neutral stimuli, the snake slide shown briefly then another slide was shown so as to obliterate any visual memory of the first one. All of these involved SNAKES. Fear motivates the examination of atypical stimuli and motivates escape and avoidance behaviors like fleeing, hiding, or freezing.
You will not see me in the Reptile House at the Philadephia Zoo, attending a screening of "Snakes on a Plane", or rewinding to review the original "Raiders of the Lost Arc". I will be avoiding those environments.