People usually form beliefs by accepting what they've been told by someone they trust: parents, teachers, media and so on. Our beliefs can change when we learn new things. But when we become convinced of something, it is similar to a religious belief in the way our brain operates. We may react with anger when challenged. This human tendency often leads us to seek out information which confirms what we already believe and ignore everything else - it's a cognitive bias actually - called confirmation bias.
It seems obvious why confirmation bias can be a problem - it can prevent us making good decisions. It makes us rigid thinkers. Someone can easily fool us by simply appealing to our established belief systems. The good news is that there are some practical strategies to overcome this natural human shortsightedness that I'll let you in on at the end of the post.