You have no idea what people are dealing with in their personal life. So just be nice, it's that simple.
That's most certainly true especially with me. I am smiling and have a happy attitude all the time. Some people are just miserable and make life unbearable to others and enjoy doing it. There is no reason not to be nice, and if you don't like someone just stay away from them. Always strive to be nice, or just polite. No one ever gets hurt that way. we do not know what situation the others are in, therefore be nice and helpful to them because it costs us nothing.