The cover of the convention invitation has Jesus looking like he can't believe it either.
There's so much to unpack from this! I mean this is a paradigm shift.
The joyless, plodding, funereal melody of the song is a clue that watchtower wants to be clear the birth of Jesus, while being Good News that brings Joy to all mankind, is not in itself something joyous or to be celebrated. Just follow that line of thought!