It’s all about nervous system, and probably some sort of evolutionary feature; it’s not only humans that can get such a "fobia" of certain common food substance. If something very displeasant is associated to a food, the brain kind of labels it "potentially dangerous" so as to prevent the person or animal from eating it again. It is commong, for example, that if a person gets a severe stomach diease that makes them vomit, they sometimes associate it to some food they’ve eaten right before that, and do not want to eat that again. This way the body makes sure that you do not eat that potentially dangerous food again in the future.

Also, because this aversion is caused by the central nervous system, people can overcome these if they really want to. By eating very small amounts of the substance in question and slowly increasing the "dosage" it is relatively easy to learn to like something that made you almost puke before, if the reason for that was something like a childhood trauma, and the food does not in reality taste disgusting (i.e. the other people eat it without any problems).