Why are we often convinced that we're right even when we're wrong?
Why are we jealous, or paranoid, even when we have absolutely no reason to be?
Why is it so easy for fake news to spread around the globe and fool us?

It's because we don't see the world as it is, rather we reconstruct it in our mind. Reality is way too complex and multiple to be apprehended by our capacities of attention, which are quite limited, as well as our brain abilities. That is why our perception of the world is subjective and various elements influence the way we acquire knowledge and form opinions. Our brain is recreating the world in its own way - most of the time for our own good: how hard would it be if, before making a choice, we had to know about all the options available in a given situation? It would take us forever to choose an item of clothing in a store, or a meal in a restaurant! Luckily, our brain can estimate: even if it makes us imperfect and subject to illusion, delusion and error, it allows us to reconstruct the world as we know it, and live in it.