With joy you will draw water from the wells of salvation. ~ Isaiah 12:3
The joy of advent is Jesus coming to earth being born as a human baby.
The angel said to them, “Do not be afraid. I bring you good news that will cause great joy for all the people. Today in the town of David a Savior has been born to you; he is the Messiah, the Lord. Suddenly a great company of the heavenly host appeared with the angel, praising God and saying, “Glory to God in the highest heaven, and on earth peace to those on whom his favor rests.” ~ Luke 2:10-14
Why were the angels joyous? Jesus had to leave heaven and come to earth to face hardships and death.
They were full of joy because it was God's delight to send his son in human form so that we might be reconnected to heaven. There's joy for everyone in that.