One of the major themes in Divergent is the concept of fear, its origins, and ways to overcome it. The initiates spend the vast majority of initiation learning ways to act in the face of fearful stimuli, as the Dauntless believe that courage is the key to a successful, prosperous world. But where does fear originate in our minds? Why do we fear the things we fear? And is it actually possible to overcome a fear, or will it remain with you forever? Understanding how fear works is key to understanding the Dauntless faction in general, and gives us a lot of insight into Tris's thoughts and actions throughout the course of the novel.
It's hard to pinpoint one area in the brain solely responsible for our feelings of fear, but identified most often is the amygdala. In the presence of threatening stimuli, the amygdala secretes the hormones responsible for fear and aggression - epinephrine, norepinephrine, and cortisol, all of which contribute to a fight or flight response. The amygdala works in collaboration with the hippocampus, which records the fearful event so that we will remember it and avoid it in the future.
But the origin of fear also has roots in fear conditioning. This often occurs when we're young; if we're eating, drinking, doing, or experiencing something when an aversive event occurs, we may then associate that thing with whatever happened and develop a fear of it rather than the actual stimulus. Common fears like heights, claustrophobia, and water develop in this way, from experiences associated with traumatic events. We can also adopt fears based on the fear culture and environment around us; while we may fear certain bugs because we don't come across them that often, other cultures might find them commonplace and unthreatening.
Fear has distinct evolutionary benefits, inciting a variety of physiological responses that are associated with survival. When something scares us, our eyes widen, allowing us to better see the threat. Our hearts begin to beat faster, pumping much-needed blood to the rest of our bodies, and adrenaline prepares for a fight-or-flight response. Most often we are fearful of things that are dangerous to us in some way: spiders, bears, snakes, drowning, guns, heights, which provoke feelings of fear as way of ensuring survival. Phobias develop in order to keep us away from life-threatening conditions. In addition, fear serves a social purpose as well; our fearful responses can alert others that something is wrong, so that they may act accordingly. All of these are beneficial to humans, and are the reasons why fear has endured is ingrained in human behavior.
So why, then, in Divergent, are the Dauntless determined to be fearless? The truth is that they really aren't. Instead, what Dauntless teaches its members is how to be the master of their own fears, and to act rationally in spite of the terror facing them. Fear cannot be eradicated, but it can be managed. One of the ways to conquer a fear is exposure; by being exposed to the fearful stimulus over and over again, our feelings of terror diminish; we don't see it as any less of a threat, but after so many times, we're much better equipped to deal with it and not let it cripple us. This is exactly how the Dauntless fear landscapes work; the more exposed they are to their fears, the better equipped they are to control them.