12/17/17: 3rd Sunday of Advent: Is it possible to rejoice always?

Today's reading from the 1st Letter of Paul to the Thessalonians has Paul making a very bold and seemingly unreasonable command: "Rejoice always. Pray without ceasing. In all circumstances give thanks." What does Paul mean when he says, "Rejoice always"? Is that to pretend that at times in life we do not experience great sadness and heartache? The Christmas mystery -- the Incarnation of the Son of God -- helps us to understand how it is indeed possible to "rejoice always".