Fear

Our brain enables us achieve amazing things, but evolutionarily speaking, it's really a kludge, a mess thrown together by millions of years of evolution. A good analogy of brain is an ice cream cone and it has three scoops: the very bottom one takes care the vital biological activities that keep us alive, heart beat, breathing, etc. The next scoop is our limbic system, it's quite primitive but very quick and powerful. The top scoop is what makes us human -- the frontal cortex where logic and reasoning happen, but it's also slow and could be easily overruled by the limbic system.

This is from Newsweek:

The evolutionary primacy of the brain's fear circuitry makes it more powerful than the brain's reasoning faculties. The amygdala sprouts a profusion of connections to higher brain regions—neurons that carry one-way traffic from amygdala to neocortex. Few connections run from the cortex to the amygdala, however. That allows the amygdala to override the products of the logical, thoughtful cortex, but not vice versa. So although it is sometimes possible to think yourself out of fear ("I know that dark shape in the alley is just a trash can"), it takes great effort and persistence. Instead, fear tends to overrule reason, as the amygdala hobbles our logic and reasoning circuits. That makes fear "far, far more powerful than reason," says neurobiologist Michael Fanselow of the University of California, Los Angeles. "It evolved as a mechanism to protect us from life-threatening situations, and from an evolutionary standpoint there's nothing more important than that."