Happy/sad paired masks are referred to as the comedy/tragedy masks or as Greek theater masks. They represent the theater and refer to the range of emotions presented by stage actors.
Dating from at least the time of Aeschylus, Greek theater used full face or head masks as standard acting props. Actors donned these masks to become the characters they were playing. Over time, this convention gave way to less restrictive masks representing only emotions. Of these, the comedy and tragedy masks survived to become a symbol of the theater.
The comedy mask was associated with Thalia, the Greek muse of comedy and bucolic poetry, and the tragedy mask is associated with Melpomene, the muse of tragedy. Upon completion of a play, Greek theatrical masks were placed on the altar of Dionysus, patron god of the theater.