It's possible that it may just be innate in some way, there have been studies to try to see if people can be classically conditioned to develop adverse responses to different stimuli and snakes, spiders, things like that are easy to condition while other stimuli such as flowers. It is possible therefore that the aversion we witness in others may be due to evolutionarily influenced aversion. Ever notice how people tend to be deathly afraid of spiders but not so much with cars or electricity? Anyway, just food for thought.