Why are we so mean to one another?

People are so mean.You never know what someone might be going through. For example...I work in the retail business. I'm usually a very nice person...always smiling, etc. Well 3 days ago I had my wisdom teeth surgically removed. I was forced to go back to work sooner than I should have. It was hard for me to greet people..my face is visibly swollen and bruised and I was in pain from having to talk all day. Well sure enough I had some lady cuss me up and down because she thought I had an attitude with her. She didn't give me a chance to explain... She reported me to my manager for goodness sake then she stood there looking dumbfounded when my manager told her about my surgery. Why do we jump to conclusions? Why do we have to be so rude?