It is probably true! Nothing works as a better pick me up than having a stranger smile at you. Why is that? It has do with the fact that another human being is acknowledging YOU. Someone sees you and that makes them happy. That is why seeing friends makes you smile; coming home to your family makes you smile. Because in that moment, their presence in your life means much to you.
The Prophet (pbuh) used to say:
"Every act of goodness is charity."
Sahih Muslim
This is why smiling is regarded as one of the best forms of da'wah. A smiling face represents a kind disposition, displays confidence, and makes you more approachable. It also puts others at ease, making them more inclined to hold a conversation. Our beloved Prophet Muhammad (pbuh) spread Islam and created his Ummah from his inviting character, beginning with his beautiful smile!
It was narrated by Abdullah ibn Haarith:
"I never came across a person who smiled as much as the Prophet (pbuh)."
At-Tirmidhi
And Jarir ibn Abdullah said:
“I did not see the Prophet, since I embraced Islam, without a smile on his face. I saw him smiling when he could not see me and he would smile in my presence.”
Sahih Bukhari
So you see, smiling is one of those actions that helps you and helps others. So why not smile?
Click the link to read more about the benefits of smiling and the hadith behind the sunnah:
http://dailyhadith.adaptivesolutionsinc.com/hadith/Give-a-Smile-Instead-of-Chocolate.htm