We humans evolved in order to survive. Throughout the bulk of our history, darkness has been dangerous. Predators, stepping off a cliff by accident, predators, enemy tribes, and predators. And did I mention predators?
Natural selection caused those who feared the dark to stay cowering in their houses at night, while those who did not went hunting in the middle of the night. The latter group died very often while out on their nocturnal expeditions, so were able to have less offspring. Those that feared the night overran the population, because they were better equipped to survive. A similar process would have caused the widespread appearance of many other fears, including the fear of heights.
So many have thought fear a weakness, but it can also be a strength, depending on what it is you fear and whether it poses actual danger to you, directly or indirectly.
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