Rejoice in the Lord always, and again I say Rejoice (Philippians 4:4). All our joy must terminate in God, and our thoughts of God must be delightful thoughts. Delight Thyself in the Lord (Psalm 37:4)…Observe, it is our duty and privilege to rejoice in God, and to rejoice in him always; at all times, in all conditions, even when we suffer for him, or are afflicted by him. We must not think the worse of him or of his ways for the hardships we meet with in his service. There is enough in God to furnish with matter of joy in the worst circumstances on earth…Joy in God is a duty of great consequences in the Christian life; and Christians need to be again and again called to it.
– Matthew Henry
as quoted by Piper, John; When I Don’t Desire God: How to Fight for Joy; copyright 2004; Crossway Books; Wheaton, Il.; p. 48