I would like to hear some input on this.
I think that humans fear things for two reasons.
1. It is unknown, foreign, or misunderstood by the person. Humans by nature prefer things that are familiar that we know will not harm us. This goes even to a level of knowledge. The more we know about something the more comfortable we are with it. For example, if you know absolutely everything about the 2005 Chevy Silverado, you will tend to keep that car longer than a car you know little about, you will make larger, more detailed and accurate responses to questions about it, and you will cling to that knowledge more than fleeting thoughts or less known information.
Now take that same truck, replace the engine, the radiator, tires, transmission, steering, repaint it, re-do the interior, and change the name. Now you know very little to nothing about it. Do you still love it as much, care about it, cling to its knowledge? Of course you don't because now its foreign to you. Do you fear it? Not really, you know its just a vehicle. But if you replace it with a random object covered in Darkness, are you at least apprehensive?
2. Survival instinct. This one is simple, if we believe something poses a threat to our survival, whether founded in fact (Fear of Spiders, Fear of Heights) or not (Fear of Clowns, Fear of the Dark, Fear of Long words) we will naturally want to avoid it at all costs.
tl;dr Answer the damn question.