I think that smiling at other people draws them in and often elicits a positive response. And smiling can lift our mood. I don't know how it works but it does.
I suppose when you see another person looking miserable or bad tempered you feel wary about approaching them and feel you have to tread on eggshells if you talk to them, but if they're smiling you can be more relaxed with them.
So let's face the day with a smile folks!