Leon Festinger published one of the most influential papers on human behavior in 1957. Festinger, a psychologist at Stanford University, published Theory of Cognitive Dissonance in it he unfolded a very simple theory that can help explain very complex reactions and actions.
Cognitive Dissonance is not a complex theory. In fact, it can be easily described as being simple. Basically, everyone holds different sets of ideas, thoughts, and beliefs. These are also known as cognitions by scientists. These cognitions are distinct, for the most part, and unrelated to each other. Just like having a deep liking to fishing has nothing to do as to who won the election in 2004. However, when our thoughts, cognitions, or actions are in fact related to each other, we feel a deep desire and necessity for them to be consistent.
Having contradictions or contradictory cognitions cause a state of dissonance that the mind can't tolerate. The contradictory cognitions or behavior must change in order to bring back the brain into a state of equilibrium. Therefore, it's considered easier to change our thoughts rather than behavior, to alter our mindset.
One example of cognitive dissonance, that Festinger used, was smoking. For example, a smoker experiences cognitive dissonance when they hear about the health risks. They could change their dissonance by stopping their behavior and quit. However, because behavior can be difficult to change, the smoker has a higher probability of changing their way of thinking in order to reduce the stress that comes with dissonance. They might look at the positive aspects that come with smoking like reducing stress or tension and helping to suppress hunger for weight management.
The smoker could argue that to quit smoking could result in weight gain which can also be considered unhealthy. Also, they could compare the dangers to unrelated dangers like driving a car and car accidents. These rationalizations help people keep their beliefs consistent with their behavior: thus, reducing cognitive dissonance.
Other information
Military hazing works on the concept of cognitive dissonance. Studies prove that the more difficult or strenuous initiation ceremonies are, the more loyalty and unitiy the new members will feel. This is known as the effort-justification paradigm by social psychologists.
Buyer's remorse can be explained by cognitive dissonance.
Sources and further readings
Yale University
Harvard University
Simply Psychology
Wikipedia
Harvard Kennedy School