If people thought completely rationally when told arguments on two sides, they would remember the most plausible. Dissonance theory predicts that instead they will remember the most plausible arguments from the side they agree with and the most implausible from the one they disagree with. This is not rational but confirms their beliefs and prevents dissonance. 

University of Michigan - Ann Arbor

Much of human thought and behavior isn't rational, but rationalizing. When people are cognitively dissonate, they spend a lot of time convincing themselves that they are right. 

Rational v. Rationalizing in dissonance theory

Aronson, Elliot, and Joshua Aronson. The Social Animal. Worth Publishers, Macmillan Learning, 2018.

The Social Animal

Arguments under dissonance theory

After making decisions, people are more likely to rate their decision as positive even if no change in circumstance has occurred other than the decision. For example, when people are told to value different appliance, they rate the appliance higher after they given that appliance as a gift, even if they have not yet used it. Another way to think of this is that people rationalize past decisions with their judgements. 

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