Here's an interesting concept:

Cognitive Dissonance
A state of opposition between cognitions (in this case defined as an attitude, emotion, belief or value). In brief, the theory of cognitive dissonance holds that contradicting cognitions serve as a driving force that compels the human mind to acquire or invent new thoughts or beliefs, or to modify existing beliefs, so as to minimize the amount of dissonance (conflict) between cognitions.

e.g. an experiment in 1957 concluded that human beings, when asked to lie without being given sufficient justification, will convince themselves that the lie they are asked to tell is the truth. Only when sufficient justification is given, researchers speculated, are human beings able to resist having their mind instantly reprogrammed by any request that they lie.

I think a simple example would be say you go out and buy a new car or a new refrigerator. And the next day you're reading the papers and you find you can get a similar or better one for the same price or even cheaper. One reaction would be to try and rationalize why you are still better off, e.g. finding faults with the new offer (not as good warranty, dealer not as reputable, etc). I know I would do that. We always try to rationalize our actions.

Explains religion maybe? Read more at wikipedia