The shepherds had been told all their lives they didn't belong, especially in the presence of God. The angels had an invitation to the shepherds that a savior had been born to them and they were welcome to meet him. This is why the angels burst into song and its the reason the shepherds went home praising God. God welcomes us, despite who we are or what we've done. He wants us to come close to Him, which is all the reason we need to celebrate!