My grandmother used to tell me often as a child that you should always smile at people when you greet them.
"You never know," she'd say. "What if you're the only person who smiles at them all day long?"
Might sound cheesey—but I try to remember that to this day. Every day.
And when I remember that, I inevitably think of this famous quote:
"Be a rainbow in someone else's cloud." - Maya Angelou