I often wish that the imagery of Epicurus had more "warmth", however smiling in portraiture is a late modern phenomenon which came about following advances in photographic technology.
Back in ancient times, for the likeness to be captured correctly I would imagine that people would sit and pose for the sculptor.
In early photography the subject had to hold still for a certain length of time.
Mark Twain had this to say: "A photograph is a very important document, and there is nothing more damning for posterity than a silly smile captured forever."
Likely people in ancient Greece felt the same (about sculptures), and this article sheds further light: