One of the more recent perspectives to have guided our understanding of psychology and human behavior has been evolution and natural selection, and one very important theory derived from an evolutionary perspective is the theory of evolutionary mismatch. I think this is worth presenting at a layman level because it answers so many questions about why on earth we behave in such weird ways.
Dan Ariely's book, Predictably Irrational, outlines an array of seemingly irrational ways that humans behave. A more comprehensive list of such irrationalities, technically known as cognitive biases, can be found here: http://en.wikipedia.org/wiki/List_of_cognitive_biases. Cognitive biases distort the way we perceive the world away from its objective features towards some other (incorrect) perception or interpretation of it.
Why do these psychological biases happen? The environment we live in today is drastically different from the environment we used to live in during ancestral times. Ancestral environments were particularly harsh, and many events were significantly a matter of life or death (and more accurately, a matter of reproduction or failure to reproduce). Harsh environments exert greater selection pressure on its inhabitants, since only those who survived could pass on their genes. Today's world is relatively secure and safe compared to the past and, as such, changes in our human nature are less likely. Additionally, our world has developed extremely rapidly such that our genetic makeup has not caught up. Our skulls therefore house stone-age brains, as described by Leda Cosmides and John Tooby. Our mind (and body) was forged through the trials and tribulations of an ancient world, and has remained largely suited for that ancient world, while the world itself changed to become more technologically advanced. As a result, we are still more afraid of snakes than cars and electric sockets, even though we're less likely to encounter and get killed by snakes than cars and electric sockets. In the psychological realm of human mating, women still care very much about how much a man earns (men don't care as much about how much women earn), despite the fact that many modern women today are capable of being financially independent, because women are still adapted to an environment where gaining the provision and protection of a resourceful mate increased her chances of survival.
Quite importantly, these biases suggest to us that we were not evolved to know the truth. We could, in principle, seek the truth consciously (which is what scientists do vocationally). However, the senses we are equipped with do not directly lead us to perceiving reality accurately; rather, they bias our thoughts and behavior in a way that best maximizes survival and reproduction. For instance, in a harsh ancestral environment, "losses" (such as injury or loss of food) may soon lead to death or failure to reproduce; people are therefore adapted to generally be more sensitive to losses than gains. Advertisements that tell consumers what they stand to lose are more effective than those informing consumers what they stand to gain. If a certain delusion enables an individual to survive and reproduce better, then that inaccurate, indeed irrational, way of perceiving the world will be more likely passed down into future generations. Rational truth is a luxury of our modern world.
We evolved in an outdated environment, but today's world doesn't facilitate the updating of many of the psychological traits that we have. Our irrationalities are a result of the mismatch between today's world and the ancestral world that our ancestors evolved in. And we were evolved to survive and reproduce first and foremost; truth-seeking comes later.