This is called confirmation bias.
Once we believe something, information supporting that belief tends to stand out.
Information that disagrees may get questioned more carefully, forgotten more easily, or explained away.
The brain likes information that fits
Suppose you believe a certain car brand is unreliable.
Every time you see one broken down, your brain says, “See? I knew it.”
You probably do not count the hundreds you passed that were running normally.
The confirming example is memorable because it fits the existing belief.
Contradictory evidence requires more work
Information that clashes with a belief creates tension.
The brain has to decide whether the evidence is wrong, whether the belief is wrong, or whether some exception explains the mismatch.
That takes more effort than simply accepting a familiar pattern.
Confirmation bias is often automatic
People do not usually wake up and decide to distort evidence.
The process can happen quietly.
We choose sources we trust.
We notice examples that fit.
We remember arguments that feel right.
The overall pattern can reinforce the original belief.
Does everyone do this?
Yes, to some degree.
Expertise does not make a person immune.
Neither does intelligence.
In fact, being good at reasoning can sometimes make someone better at defending the position they already prefer.
How can you fight confirmation bias?
Ask what evidence would change your mind.
Look for strong arguments from the other side.
Separate the quality of evidence from whether you like the conclusion.
Those steps are uncomfortable.
That is exactly why they are useful.
Does changing your mind mean the original belief was foolish?
No.
Good reasoning includes updating beliefs when better evidence appears.
The goal is not to never be wrong.
The goal is to become less wrong when new information deserves it.
One good defense is to deliberately ask, “What evidence would convince me I am wrong?”
That question is annoying.
Which is probably why it is useful.
A related piece of human behavior appears in Why are first impressions so hard to change?, where the brain handles another everyday judgment or response.
And that’s the short explanation.
Related explanations: Why are first impressions so hard to change?, Why does confidence make someone seem believable?, and Why do things seem obvious after they happened?