People's behavior is driven kind of by their emotional needs, rather than any sort of utilitarian economic man theory. We seem to have this extraordinary instinct where feelings where we have not enough information to make decisions make us very unhappy. There may be a very good reason why we want a lie - if we had fuller introspective access to our motivations, we wouldn't be able to deceive both other people or ourselves. A degree of self deception is absolutely necessary in our ability to market ourselves to others.