Essentially, anyone can become afraid of anything at any time for no apparent reason. Still, people try to put a label on horror to help categorize specific types of media that should typically bring forth feelings of dread and fear in the large percentage of the population. The most likely reason to find something horrific is the careful consideration of the unknown. We're afraid of the monsters, killers and other people, because we do not know what their intentions are. Darkness is frightening, because we don't always know what is lurking within the thick blackness. Death is one of the scariest thoughts someone can consider, because we do not know what will happen to our loved ones when we are gone, and we do not know if anything is waiting for us beyond this world. This fear of the unknown is derived from the dreadful thoughts that something horrible may occur from the given scenario.
If you consider the probability that anything could go wrong with any given scenario, we would constantly live in fear, but most are able to muster enough courage ignore the odds and live a normal life. Still, placing a label on what can and cannot be considered horror is both illogical and impossible. Doing so isn't quite so different from making fun of another kid for being afraid of something back when we were children ourselves. As adults, we probably won't make another person cry for disagreeing with what they believe is horrific; however, the basic concept is still the same. It's best to keep an open mind and listening ear for someone's feelings of fear, because everyone is afraid of something.