One of the most important theories in social psychology: the idea that people are motivated to reduce cognitive dissonance by either changing their attitudes and behaviors, or justifying and rationalizing them.

Example: Bombarded with anti-smoking advertisements on television, a lifelong smoker grows increasingly concerned for her health but is unable to quit. She rationalizes that smoking has helped her to maintain her weight, so it can’t be all bad for her.

I think this site provides an interesting overview of the theory: http://changingminds.org/explanations/theories/cognitive_dissonance.htm