Apparently, telling lies is a deeply rooted evolutionary trait instilled in our DNA for a very long time. The book covers a variety of ideas on why we are so good at telling lies, including a long section about self-deception and some ideas on why we are so good at hiding information from ourselves.
Interesting tidbit: The average person tells three lies every ten minutes, many of the lies the individual would not consciously recognize that they were telling a lie at the time.