Something about the arrival of Jesus results in steady, unfaltering joy that triumphs through the difficulties of life. As the angel explained, Jesus' birth is good news of great joy for all of us.
See in this passage how the character of God - specifically His saving love - is the bedrock that produces steady joy in a person's life.
Jesus' birth reminds us of the powerful presence of God - that with God, nothing will be impossible. This is joy for the world! Nothing can stop God from keeping his gracious and glorious promises.