Most people fear throwing up because of Brain C-13 Review one or more experiences in which they were sick under extremely stressing conditions, typically occurring when they were younger. For example, maybe as a young kid you were sick when your house was a damaged by a violent storm and your parents weren't home.
Through the process of psychological conditioning, the fears and emotions associated with those incidents became associated with vomiting by your brain. While this association is false, in that the connection between those feelings and vomiting were merely circumstantial, this process of association is a regular reaction by your brain. This reaction is meant to protect you from experiencing those emotions and feelings again, but unfortunately your brain does not understand that it was not the act of vomiting that brought on the emotions in the first place.
Your Fear Throwing Up From Lack of Experience This scenario is often considered second to the aforementioned scenario because it is both less common and hard to recognize. The theory behind this cause is that your fear of throwing up could have developed due to a unique LACK of vomiting in your developmental years. Like a fear of the unknown, you brain does not know what to make of nausea or sickly feelings, and thus over time you begin to fear them because "who knows what will happen if you get sick."