This scene with the shepherds and the angel mass choir is depicted every year by adult actors on stage and by little children in pageants, trying to strike the devout poses from a creche. But let's step out of the world of the cute and the dramatic for a moment, and step into what happened with the shepherds.
Shepherds were often newcomers to the land or hired hands from lower classes. They were not at the bottom of the social order -- it wasn't seen as unholy -- but it was a hard life facing dirt and weather for little money. They lived out in the fields all year, and had little opportunity to leave the flock because of the ever-present dangers of predation and theft. They were just doing their usual work that night -- after all, they weren't yet being given a day off for Christmas.
Why did the angels bring the tidings of great joy to these shepherds? Luke doesn't say. It may have had something to do with their poverty, but there were other poor people in the area, even many other shepherds. We don't even read about how many shepherds were there; it could be any number from two on up. As for the heavenly multitude, well, they were strutting their stuff and putting on a party. I guess the shepherds knew that when a heavenly multitude says something's important, it is. So they ran off to see this baby. Let the angels watch flocks for a while!
The infant Jesus had already come out before the shepherds were told, so he was laying there in his feedbox crib. When the shepherds arrived and saw that the choir wasn't joking, they understood that the rest of what they were told was true too : a Savior, a Messiah, their rescuer, had come. God has taken action, and come into their presence. So now they started picking up on the angels' carol, praising God to all who were nearby along the way back.
God came to us as a baby, died as an adult, lives as God-With-Us. We can still pick up on the choir's lyrics. Join the party!
Almighty God, you made yourself known to us in your Son Jesus, starting as a baby just like we did. Lead us from our baby faith to Christ-likeness, that we may live fully in the freedom Christ has given us. Amen.