Cognitive Disssonance Theory

The cognitive dissonance theory was developed by Leon Festinger. In this theory one should think of cognition as a 'piece of knowledge'. Cognitive dissonance is an uncomfortable feeling caused by holding conflicting perception simultaneously. Cognitive dissonance proposes that people are motivated to reduce dissonance. They do this by changing their attitudes, beliefs, and actions or in short changing the way they act. Dissonance is also reduced by justifying, blaming, and denying or being in denial. It is the most popular theory in social psychology. Cognitive dissonance can also be referred to as a feeling of uncomfortable anxiety which comes from having two contradictory perspectives of the same thing at once. One can decrease this anxiety though, by changing their behavior towards something, justify their actions toward the cognition that they do not favour, and/or add a new idea to the cognition to make it seem better.
One's dissonance increases with the importance of the subject or something to them, how strongly the dissonant thoughts clash with one another and lastly one's inability to rationalise and clarify away the conflict of ideas.

Examples
Someone may take home office supplies they know it's wrong and can get in trouble if caught, but they ease their feelings of doing anything wrong by justifying that they don't get paid enough, or that there are so many surplus supplies lying around wasting.
Someone you dislike greatly does something very generous towards you...it makes you feel awkward and weird because you dislike that person, maybe you write it off as them doing it on purpose to make you feel bad for hating them!
Someone knows that it is very risky to over speed and might cause an accident or even be fined, but he might over speed just because he's late. People often take risks to fulfill their wants.