The angel who visited Mary, this young lady who is engaged to Joseph, came to bring her the good news that she will be pregnant and will give to the world a son whom we call Emmanuel, that is to say, God with us. The birth of Jesus Christ was a subject of joy for all nations according to Isaiah's prophecy. There was a joy not only on Earth but first in heaven, the shepherds, the Angels were in joy at the announcement of the news by the Angel Gabriel. The Angels returned to heaven singing and praising the Creator because the Savior had just born.
This is the same joy that is felt when a sinner repents. It is the image of the new birth that occurs in the heavenly government. Christ illustrates it in the parable of the lost sheep. There had a party when this sheep was found.