John Piper, Desiring God, pg.93-94:
YOU MUST KISS ME, BUT NOT THAT KIND OF MUST
Consider the analogy of a wedding anniversary. Mine is on December 21. Suppose on this day I bring home a dozen long-stemmed roses for Noël. When she meets me at the door, I hold out the roses, and she says, “O Johnny, they’re beautiful; thank you” and gives me a big hug. Then suppose I hold up my hand and say matter-of-factly, “Don’t mention it; it’s my duty.”
What happens? Is not the exercise of duty a noble thing? Do not we honor those we dutifully serve? Not much. Not if there’s no heart in it. Dutiful roses are a contradiction in terms. If I am not moved by a spontaneous affection for her as a person, the roses do not honor her. In fact, they belittle her. They are a very thin covering for the fact that she does not have the worth or beauty in my eyes to kindle affection. All I can muster is a calculated expression of marital duty.
Here is the way Edward John Carnell puts it:
Suppose a husband asks his wife if he must kiss her good night. Her answer is, “You must, but not that kind of a must.” What she means is this: “Unless a spontaneous affection for my person motivates you, your overtures are stripped of all moral value.”6
The fact is, many of us have failed to see that duty toward God can never be restricted to outward action. Yes, we must worship Him. “But not that kind of must.” What kind then? The kind C. S. Lewis described to Sheldon Vanauken: “It is a Christian duty, as you know, for everyone to be as happy as he can.”7
The real duty of worship is not the outward duty to say or do the liturgy. It is the inward duty, the command: “Delight yourself in the LORD”! (Psalm 37:4). “Be glad in the LORD, and rejoice!” (Psalm 32:11).
The reason this is the real duty of worship is that it honors God, while the empty performance of ritual does not. If I take my wife out for the evening on our anniversary and she asks me, “Why do you do this?” the answer that honors her most is “Because nothing makes me happier tonight than to be with you.”
“It’s my duty” is a dishonor to her. “It’s my joy” is an honor. There it is! The feast of Christian Hedonism. How shall we honor God in worship? By saying, “It’s my duty”? Or by saying, “It’s my joy”? Worship is a way of reflecting back to God the radiance of His worth. Now we see that the mirror that catches the rays of His radiance and reflects them
back in worship is the joyful heart. Another way of saying this is to say
The chief end of man is to glorify God
enjoying Him forever.