Here are some thoughts from John Piper's free ebook on Advent called "The Dawning of Indestructible Joy."
"One of the most famous Christmas scenes in the Bible is the announcement to the shepherds by an angel that the Savior is born. And then it says, “Suddenly there was with the angel a multitude of the heavenly host praising God and saying, ‘Glory to God in the highest, and on earth peace among those with whom he is pleased!’” (Luke 2:11–14).
Glory to God, peace to man. The angels are sent to make something crystal clear: the Son of God has come into his cre- ation to display the glory of God and to reconcile people from alienation to peace with God. To make God look great in sal- vation and to make man glad in God. "