Have you ever noticed how some people, when they are a customer, feel like they have a license to be a jerk? These are the people who might seem nice enough until the waitress or cashier makes the smallest mistake, then they shred the person. I think they feel it's their duty to reprimand them, as a parent would do to a child.
Our actions have a ripple effect like a rock splashing into a pond. Remember that when you interact with other people. You have an effect on them that spreads to the next person, then the next, and so on. If you help one person have a good day, you have helped countless others as well.