If you WANT to find bugs, you are more likely to find bugs / if you adopt a nicey-nicey attitude and just want to help the programmers demonstrate that their program works, you’re going to miss a lot of bugs. It’s not just that you will not report them. You’ll just not see them.
We all get biased multiple times in our lives. Biases can take many forms and they can be difficult to overcome especially when we don’t even realize that we might be biased.