When I have a good day and they are having a bad one. Or when I see something they accidentally did but didn't notice and am in a good mood. If they do something really funny, or someone around them. If they start getting upset that I'm laughing either the smile goes away or I laugh at them even more.
If I don't want to smile or not in the mood, I don't. I'm very blunt and if someone cannot laugh at themself then they cannot enjoy the hard times.