Often I believed that smiling is a natural thing that comes through happiness, and although I do believe you should let the smile come natural. However, if you try to smile naturally, your mood tends to be happier.
Today I was in my gym, and I was going through some depression. Only did I try to make myself happy and felt like I was the man. And through that I smiled. I just got a type of positive energy inside me that just kept me happy and less tired.
I am still happy. Being happy regardless of anyone that puts you down is an amazing energy for success. Think of something positive in a moment of negativity and smile.
There are scientific research and theories on this too: https://www.nbcnews.com/better/health/smiling-can-trick-your-brain-happiness-boost-your-health-ncna822591