Like other organisms, we humans make avoiding danger our highest priority. It is a simple evolutionary story: organisms that did not do so simply did not survive. Perhaps any living thing should bear in mind that it is better to miss lunch than to be lunch. Think about a bunny rabbit out in a meadow. If that bunny finds some really good veggies and becomes so engrossed in them that she does not keep looking around, she is liable to be eaten by a puma. Only bunnies that learned to eat while keeping a constant eye out for predators remain to inhabit this earth. But, unlike bunnies, we humans can describe the past and possible futures. You can think about how your father died of carotid artery blockage and worry that the same thing might happen to you. You can also worry about your child who might possibly be picked on by other kids at school. Although our thoughts about the future can make us thrilled in anticipation of good things to come, we are also prone to horrifying visions of bad things that might happen to us, thanks to our evolutionary bias to avoid danger. Then, thanks to our verbal, relational abilities, we can inhabit a terrifying world while sitting in a perfectly safe, quiet room.