Why are we all so judgmental as human beings?

We judge one another with a single glance crafting entire backstories about people we do not know and believing our assumptions to be true, running with them based off of no actual facts. 

We all do it and we do it daily, some of us try not to let those fabricated backstories take root in our minds while others readily give into those convoluted thoughts. 

Why is this so? 

Trial and Error.