Wikipedia says this about cognitive dissonance:

Cognitive dissonance is an uncomfortable feeling caused by holding two contradictory ideas simultaneously. The "ideas" or "cognitions" in question may include attitudes and beliefs, and also the awareness of one's behavior. The theory of cognitive dissonance proposes that people have a motivational drive to reduce dissonance by changing their attitudes, beliefs, and behaviors, or by justifying or rationalizing their attitudes, beliefs, and behaviors.[1] Cognitive dissonance theory is one of the most influential and extensively studied theories in social psychology.

Dissonance normally occurs when a person perceives a logical inconsistency among his or her cognitions. This happens when one idea implies the opposite of another. For example, a belief in animal rights could be interpreted as inconsistent with eating meat or wearing fur. Noticing the contradiction would lead to dissonance, which could be experienced as anxiety, guilt, shame, anger, embarrassment, stress, and other negative emotional states. When people's ideas are consistent with each other, they are in a state of harmony, or consonance. If cognitions are unrelated, they are categorized as irrelevant to each other and do not lead to dissonance.

A powerful cause of dissonance is when an idea conflicts with a fundamental element of the self-concept, such as "I am a good person" or "I made the right decision." The anxiety that comes with the possibility of having made a bad decision can lead to rationalization, the tendency to create additional reasons or justifications to support one's choices. A person who just spent too much money on a new car might decide that the new vehicle is much less likely to break down than his or her old car. This belief may or may not be true, but it would likely reduce dissonance and make the person feel better. Dissonance can also lead to confirmation bias, the denial of disconfirming evidence, and other ego defense mechanisms.

Now, cognitive dissonance as a source of amusement:

About 8 years ago, or so, I was visiting my parents in Missouri. During the course of the visit, the birth mother of my brother's son let him visit with us for a day.

Brandon was 8, and, therefore, quite a bit shorter than I am.

He walked up real close to me. He had to look almost straight up to see my face, and said: "Hi, Uncle Kevin!"

I smiled down at him, and said: "Hi, Brandon. How old are you, now?"

He beamed up at me, and said: "I'm 8, Uncle Kevin."

I kind of crinkled up my face, put my hands in my pockets, and said: "Humph, ... when I was your age, I was 9."

It was hilarious!

Imagine, if you will, driving a car with a standard transmission down the interstate at about 70 miles an hour. Now, attempt to shift that car into reverse, and slam in back into the top gear.

That is what I saw play out on this young kid's face.

And once he had slammed his brain back into the "top gear", he looked up me, grinned, and said: "Kev!"

The tone of his voice was, at once, chiding, for messing with him, and that tone that said I had met his expectations ... I accepted him. I was "me". The "me" he expected.

I scooped him up in my arms, and we hugged. ... and had a great day, together.