Cheerfulness is like music to the soul. It excites to duty. It oils the wheels of the affections. Cheerfulness makes service come off with delight, and we are never carried so swift in religion as upon the wings of delight. Melancholy takes off our chariot wheels, and then we drive on heavily. Discourses 1:377
The heart must be kept courageous and strong and lively, like an instrument which is tuned to tune all the rest, or else every grief will make thee impatient. In Deuteronomy 30:9 it is said that God rejoices to do us good, and therefore in Deuteronomy 28 the Jews are reproved because they rejoiced not in the service of God. As He loveth a cheerful giver, so He loveth a cheerful server and a cheerful preacher and a cheerful hearer and a cheerful worshiper; and therefore David saith, "Let us sing unto the LORD" (Ps. 95:1), showing, as it were, the tune which delights God's ears. Sermons, 115