I noticed a couple of "bookends" when I read our passage today.
The first one jumped out at me when the shepherds were introduced and we saw their reaction to what was happening.
 And in the same region there were shepherds out in the field, keeping watch over their flock by night.  And an angel of the Lord appeared to them, and the glory of the Lord shone around them, and they were filled with great fear.
(Luke 2:8-9 ESV)
I noticed the second one near the end of our passage where there is the last mention of the shepherds and their reaction to what happened.
 And the shepherds returned, glorifying and praising God for all they had heard and seen, as it had been told them.
(Luke 2:20 ESV)
Glorifying and Praising God = Joy.
How was the shepherd's initial fear morphed into the joy of glorifying and praising God?
They became part of a great story. A life changing story.
They had an encounter with the Savior.
Who changed lives even as he was wrapped in swaddling cloths and lying in a manger.