During my whole long career, I never understood why a painted portrait should not show cheerfulness.
Often heard reactions “A joyous portrait will be boring in the long run" or "Do not show teeth because you than go back on the fact that you worked from photography”. Why not portray someone, who is feeling good? Why not look for that split second that the sitter looks fine?
I would encourage portrait painters to ignore any so-called generally agreed practices.
My slogan is: Don´t stick stubborn to the rules.