A seller must constantly smile. Likewise, almost all people working in public relations. But if someone is not the way a smiling threads should mimic smile, to fake, to display a fake smile.
Well, that's a big problem. The false will eventually tear the soul. Constantly living this lie (to show you are joyful when in fact you're not) psychological problems will occur. In some cases depression. The truth is better than the most beautiful lie. And a life lived in truth is guaranteed safe life.
In professions who smiles is required all day, would fit perfectly people who smile all day anyway, people who are smiling from birth.