Happiness is contagious.
I truly believe this. I can be a grouchy and you encounter someone who is in a great mood and you end up in a great mood, too. I love when this happens.
I think I've done the same for other people, too. I'm a glass half full kind of person. At the very least it is half full. I'm not perky, per say, but I'm pretty upbeat.
So smile at someone it may make their day.