If you don’t have a smile while you are serving the Lord—you’re missing out on the reason to serve in the first place.
Serve the LORD with gladness: come before his presence with singing. Psalm 100:2
Have you ever been in a place that you lost the smile in your service? You begin to serve out of obligation rather than joy? You lose the desire to see souls changed for eternity? If you ever get to that point, stop, get alone with the Lord, and let Him have His way! Let Him search your heart and get to the root of the issue. Maybe it is sin in your camp. Maybe you have a problem with another Christian. Maybe someone hurt your feelings. Maybe God didn’t answer a prayer the way you wanted.
Whatever the reason is that you have lost your joy, follow David’s lead in Psalm 51:12 when He said, “Restore unto me the joy of thy salvation; and uphold me with thy free spirit.”