Life is like a mirror, we have better results when we smile. Source
That gesture that allows us to express happiness, joy, to unite through it with other people, even without knowing each other, should be practiced with more continuity.
A way of expressing ourselves that we do not learn, is simply something natural, it arises only before becoming aware that there is something called smile, which even in newborn children can be perceived.
Like Charles Gordy says:
A smile is a cheap way to change your appearance.
In general, as we become adults, we lose the ability to laugh at anything, just as we did when we were children, we are supposed to mature, take responsibility, and this of becoming more serious is implicit in decreasing the number of times we laugh.
We must not forget that the smile is a kind of good, highly contagious virus, and we must spread smiles wherever we can. Nor should we forget that:
"Sometimes your joy is the source of your smile, but sometimes your smile can be the source of your joy" Thich Nhat Hanh...
So I just have to insist that: