I regularly talk to my children about smiling. I try and remind them of the power of this most encouraging expression. I've shared with several friends, of my attempt to inspire one of my children to smile. Of course, I waited until my child ,whose name I won't 
mention, was in a rotten mood to bring up the importance of a smile. I was challenged with the words, "Do you want me to lie?". Confused, my child told me that if you're really not feeling happy, it's like a lie to smile. So I said "Yes, you have my permission to lie in that case, I want you to smile".
Honestly, a smile is not meant for one's self. My smile is not for me. Likewise my look of displeasure holds no gain for myself. How small to think that because my heart is not in the right condition, that I have the right to pain people around me with outward unpleasantness. Our shared disenchantmant can spread quickly like a virus. I believe your countenance though, much through facial expression, is your wordless, effortless, free gift to those you encounter. 
The wonderful thing about a smile is the fact that gracing someone with a grin often turns out to be a double blessing.
.A cheerful look brings joy to the heart. -Proverbs 15:30
It is when we look to grace someone else we are often distracted from our "heart condition"; forgetting our own complaint. By our kind gesture, our anger or sadness many times melts leaving ample room for joy to fill our hearts.