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Cognitive psychologists have a theory called "backfiring."
When presented with information that goes against your viewpoints, you not only reject challengers, but double down on your view. Voters often view the candidate they support more favorably after the candidate is attacked by the other party. In investing, shareholders of companies facing heavy criticism often become die-hard supporters for reasons totally unrelated to the company's performance.

So when you think you have a great idea or onto a great stock, go out of your way to talk with someone who disagrees with it. At worst, you continue to disagree with them, but in any case fight confirmation bias like the plague!