Once you’ve formed a belief, adding exceptions and justifications becomes easier than updating it.
Ryan Holiday writes about this in Trust Me, I’m Lying:
Once the mind has accepted a plausible explanation for something, it becomes a framework for all the information that is perceived after it. We’re drawn, subconsciously, to fit and contort all the subsequent knowledge we receive into our framework, whether it fits or not. Psychologists call this “cognitive rigidity”. The facts that built an original premise are gone, but the conclusion remains—the general feeling of our opinion floats over the collapsed foundation that established it.
Information overload, “busyness,” speed, and emotion all exacerbate this phenomenon. They make it even harder to update our beliefs or remain open-minded.
Charlie Munger said something similar:
[W]hat I’m saying here is that the human mind is a lot like the human egg, and the human egg has a shut-off device. When one sperm gets in, it shuts down so the next one can’t get in. The human mind has a big tendency of the same sort. And here again, it doesn’t just catch ordinary mortals; it catches the deans of physics. According to Max Planck, the really innovative, important new physics was never really accepted by the old guard. Instead a new guard came along that was less brain-blocked by its previous conclusions. And if Max Planck’s crowd had this consistency and commitment tendency that kept their old inclusions intact in spite of disconfirming evidence, you can imagine what the crowd that you and I are part of behaves like.
Once a belief is formed, the mind tends to stick to it, making it difficult to accept new ideas. Even the most intelligent people can be trapped by their previous conclusions, so it’s important to stay open-minded and avoid being influenced solely by popular opinion.
The trick, if there is one, is to form working beliefs, not permanent beliefs.