The Angels say that smiling was a gift given to humanity to help them change their emotional thermometer. It seems when the Earth was dark and the vibration very low, humans, such as they were, gave off an angry, aggressive vibration.
For mankind to progress, the Angels say, the humans needed to be able to raise their vibration to pull in the small rays of light that were beaming to Earth. The Angels pondered the problem. They tried to give humans games, songs, and other activities to raise their spirits. Nothing worked.
The Angels took their problem out to the Universe and asked what might change the humans so that they were not so angry and aggressive. The Universe, in turn, pondered the problem, and determined that a physiological solution was required. The Universe realized that the humans needed to have some kind of self worth and needed to feel tranquility, and perhaps contentment. Further, the humans needed to be able to express that they were not angry or feeling like fighting.
The Universe decided on some physiological adjustments to the human DNA. One of the mechanics placed in the DNA was the ability to change the face muscles to shift upward to show a relaxed countenance. And, the Universe shifted the DNA to include some emotion and self awareness. Over several generations, humans began to smile to show that they were content and happy with themselves and with each other.