The reason we fear things , is because we have an expected outcome when we approach it. The outcome might not be appealing, or we just don't know. I believe these are scenarios are made up in our heads and the uncertainty, scares us because we don't know what to expect. Our brain doesn't know how to react unless logic comes to us.
We fear lots of things that we know about. Like the dude above me said the fear of pain. Obviously there is other fears. I fear god exists not because i dont know whether the after life would be good but because i know living forever with a god who treats us like this would be horrible. Slaves thought their masters hadnt wronged them because they were born into it and it seemed normal. Well peopel are born into this life but if you think about it god set up our nerves and brains and decided that we would have to feel great pain. He did the one cruel thing he could do and then created hell supposedly? And hes perfect and loves us? Yea right. And if nobody is perfect then we dont have the ability to be perfect. If we dont have the ability to be perfect then we dont deserve punishment for not being perfect.
The reason we fear pain is not because we don't know what is going to happen, but because we do know what is going to happen--it's going to hurt. We fear getting hit in the head, bitten by a black widow, or our finger chopped off because we will feel pain at the occurrence. Pain may be associated with the unknown--like if we might die because of the black widow, how we are going to live without our finger, or if we will suffer brain damage from the blow to the head--but we also fear the pain in-and-of-itself. A known evil. Death and loss are unknown evils, pain and suffering are known evils.