When we do something out of duty, or for the reason only of being moral, the words to be used would be “ought” and “must” and often run contrary to “desire”. They shouldn’t, but they do. We are a sinful selfish people who must be told by God what is right and how we should live. But in Heaven, where all sinful and selfish desires will have been completely eliminated, “desire” and “duty” will come together. What we “ought” to feel and think and do will come automatically as an expression of our deepest inner desires.
No one I know of spoke more clearly of this than C. S. Lewis. He said “Here and now, prayer is a “duty,” but will not be so in heaven. He goes on to explain:
“If we were perfected, prayer would not be a duty, it would be delight. Some day, please God, it will be. The same is true of many other behaviors which now appear as duties. . . . To practice them spontaneously and delightfully is not yet possible. This situation creates the category of duty, the whole specifically moral realm. . . . But . . . there is no morality in heaven. The angels never knew (from within) the meaning of the word ought, and the blessed dead have long since gladly forgotten it. . . . This also explains . . . why we have to picture that world in terms which almost seem frivolous. . . . We can picture unimpeded, and therefore delighted, action only by the analogy of our present play and leisure” (C. S. Lewis, Letters to Malcolm pgs. 147-48).
J. I. Packer agrees with Lewis:
“When we are resurrected in our finally perfect state, duty will be a word that we will not need. Thorough going love (admiration, appreciation, valuation, gratitude, goodwill), both to our triune God and to all our glorified fellow sinners, will be the spontaneous, wholehearted, unqualified and indefatigable expression of what we now are. All the thoughts that pass through and come from our minds will have at their center praise to God, all the time and all the way. And it will be fun! – “pure, celestial, transcendent, glorious fun”. We will never have enjoyed anything so much” (Praying, pg. 117).