Imagine that you have a belief such as - 'people are generally good'. This belief causes you to notice information that confirms your belief and discount information that disputes it. See the problem?
A belief introduces a bias, a perspective, and it alters how you look at the world. You essentially see what you want to see. Having opinions, ideas and preconceptions can make you narrow-minded.
Once we determine what is important, we set out to find something that fits our criteria. But what if our values are askew? What happens when we encounter things that we don't really understand?