Thomas Watson on Christians and cheerfulness:
We glorify God by walking cheerfully. It brings glory to God, when the world sees a Christian has that within him that can make him cheerful in the worst times; that can enable him, with the nightingale, to sing with a thorn at his breast. The people of God have ground for cheerfulness. They are justified and adopted, and this creates inward peace; it makes music within, whatever storms are without.
If we consider what Christ has wrought for us by his blood, and wrought in us by his Spirit, it is a ground of great cheerfulness, and this cheerfulness glorifies God. It reflects upon a master when the servant is always drooping and sad; sure he is kept to hard commons, his master does not give him what is fitting; so, when God’s people hang their heads, it looks as if they did not serve a good master, or repented of their choice, which reflects dishonour on God.
As the gross sins of the wicked bring a scandal on the gospel, so do the uncheerful lives of the godly. ‘Serve the Lord with gladness.’ Your serving him does not glorify him, unless it be with gladness. A Christian’s cheerful looks glorify God; religion does not take away our joy, but refines it; it does not break our viol, but tunes it, and makes the music sweeter.