Humans are scared of stepping outside comfort zones. Dead bodies can be peaceful to be with. It all depends on your relationship with the deceased persons. Imagine yourself in different scenarios - war - holocaust - heart attack of neighbour - etc. Once you have been around the dead you realise it is the living who truly are scary. read more
Another factor may be that seeing a dead human would raise the fear of oneself dying. One more factor can be a fear triggered by the thought of someone we know dying. So i guess, human death may trigger all sorts of fear because we can relate to another human being including suffering and death rather easily. read more