I once wrote that the subtlest of all sins is ‘unjoy.’ It seems strange at first… how can not being joyful be bad?
In order to untangle this we simply need to look at the nature of sin. The first part is obvious: doing things we’re not supposed to do (cheating, lying and stealing etc.). The second is more subtle: not doing things we’re supposed to do. We see this second aspect of sin talked about in the book of James…
James 4:17 Anyone, then, who knows the good he ought to do and doesn’t do it, sins.
So back to the original assertion… that unjoy is a sin. This seems like it would follow only if having joy was something we ought to do (per James’ definition).
So is having or manifesting joy something we ought to have or be committed to? Let’s look at a few passages.
Romans 14:17 For the kingdom of God is not a matter of eating
and drinking, but of righteousness, peace and joy in the Holy
James 1:2-3 Consider it pure joy, my brothers, whenever you face trials of many kinds, because you know that the testing of your faith develops perseverance.
Philippians 4:4-7 Rejoice in the Lord always. I will say it again: Rejoice! Let your gentleness be evident to all. The Lord is near. Do not be anxious about anything, but in everything, by prayer and petition, with thanksgiving, present your requests to God. And the peace of God, which transcends all understanding, will guard your hearts and your minds in Christ Jesus.
1 Thessalonians 5:16-18 Be joyful always; pray continually; give thanks in all circumstances, for this is God’s will for you in Christ Jesus.
Without giving much explanation to the verses above or pulling in another dozen examples, it does seem like we can conclude with C.S. Lewis that “It is a Christian duty, as you know, for everyone to be as happy as he can.” (1)
There is no doubt that unjoy is a subtle thing — most of us don’t look in a mirror often enough to see it. Whether it is a sin or not I’ll leave up to you to decide.
 C.S. Lewis in a letter to Sheldon Vanauken. Sheldon Vanauken, A Severe Mercy (New York: HarperCollins Publishers, 1987), 189.