Get the ugly out

Why do people feel they need to be rude to others to get their point across?  I like the saying "Don't judge a person til you walked a mile in their shoes."  Before you think about being mean to a person, think about what they might be going through. 
I heard a sermon once that stuck with me on this very topic.  A Pastor was at the grocery store and the checkout girl was foul.  He tried to stay patient, but eventually got mad and snapped back at her.   He went on with his day but couldn't help feeling guilty about his actions.  He regretted how he responded and went back and gave her a gift and apologized.  She cried and told him all the horrible things that were going on in her life and that day he came in some heavy stuff went down. 
I have been to restaurants on more than one occasion where people have purposely been rude to their waiters.  I believe it was their goal to get a free meal.  I remember one occasion where I watched an entire table leave zero tip.  If you don't like someone's service, don't be a hater.  In similarity, no boss would withhold a paycheck because someone had an off day. 
Can't we all just get along?