Joy to the world. It's not just a song, it's the message of the angels to those shepherds who were frightened. The jubilant proclamation was to reduce the fear that people may have when they think about God and judgment.
When Jesus came to the world, it was lost and separated from God. When He left the world and returned to the Father, He went to be our Advocate, forever interceding at the Father's right Hand as our defender. "Who is to condemn? Christ Jesus is the one who died—more than that, who was raised—who is at the right hand of God, who indeed is interceding for us." Romans 8:34.
It is the promise of heaven that gives us joy. This world is marvelous, beautiful, and can be joyous. But it is only a glimpse of eternity, a small foretaste of great and eternal joy. Jesus said when we go through hard times, "Rejoice in that day, and leap for joy, for behold, your reward is great in heaven..."
Christmas is placed on Dec. 25 for one reason. The days are only getting brighter. For six months, it's been slowly getting darker and darker, but now Joy comes in the morning. "For God's anger is but for a moment, but his favor is for a lifetime. Weeping may tarry for the night, but joy comes with the morning." (Psalm 30:5).
I don't know if it was intentional or not, but Luke begins with joy and ends with joy. John the Baptist leaped with joy at the greeting of Mary, and the final line of Luke's gospel says, "While he blessed them, he parted from them and was carried up into heaven. And they worshiped him and returned to Jerusalem with great joy, and were continually in the temple blessing God."
Merry Christmas, INDEED!