Why do people so readily judge on assumptions?

It's a thing that our brains evolved to do to help us survive. If you've been beaten up several times by tall hulking blond guys in football jerseys, your brain is going to go on the alert when it sees people with any collection of those qualities, even if they're not acting hostile. Similarly, if you were saved each time by short skinny black-haired girls in field hockey uniforms, you're probably going to be drawn towards people who look like that because you feel safe and protected around them. Also, there is a lot of information to process in our modern environment--far more than our ancestors had to deal with. It's just easier and less stressful on our brains to make quick judgments.

Sometimes people are too afraid to ask for the truth! So speculation occurs and they make an assumption and act upon it faultily.

They probably so readily judge on assumptions so quickly because they have been through that situation before. Someone might've used that line on them and betrayed them, making them more cautious.

Well, That's human nature.. they never follow "Don't judge a book by its cover". But we are not here to clear their judgements.. if they are assuming things its not our fault...and our lives are none of their business.. if you want to be happy.. then never care about what people are thinking about you.. because people are never satisfied. You are here to live your life.. not wasting it on other people. Just be yourself.

Because it's easier to come up to a conclusion based on someone else's views than to think for yourself and really try to understand a different perspective. Don't take it personally, there are people out there who put effort in how they think and those are the ones who matter for real.

The easy answer? because it is easy to let your brain sit one out when it comes to having to analyze and consider new scenarios and judgments based on assumptions are hand in hand since assumptions aren't based on finding truth but what you want to connect to be true and then based on the assumption you are easily placed with a piece of information already coloured by your perspectives and therefore judgment comes naturally to those who use assumptions