Eros the melter of limbs...sweetbitter, smiles.

Carvaggio painted Eros as a young boy with a smile, signifying his destructive potential. "By the 17th century in Europe...it was a well-established fact that the only people who smiled broadly, in life and in art, were the poor, the lewd, the drunk, the innocent, and the entertainment." For more:  http://www.theatlantic.com/technology/archive/2013/09/why-didnt-people-smile-in-old-portraits/279880/

More on the rarity of smiling in art: