I'm not suggesting that you get yourself arrested grinning inanely at every passer-by, nor that you pretend you are happy when you are not, but when you greet someone, give them a genuine, friendly smile to make them feel they count and somebody cares about them. Smiling will make you feel better and cheer up the person you smile at. Smiles can be infectious, too, soon everybody could be smiling just because of your one smile.
“If in our daily life we can smile, if we can be peaceful and happy, not only we, but everyone will profit from it. If we really know how to live, what better way to start the day than with a smile? Our smile affirms our awareness and determination to live in peace and joy. The source of a true smile is an awakened mind.”
Thich Nhat Hanh, 'Peace Is Every Step: The Path of Mindfulness in Everyday Life'