NIV – “All the days of the oppressed are wretched, but the cheerful heart has a continual feast.”
GNT – “The life of the poor is a constant struggle, but happy people always enjoy life.”
NKJV, KJV – “All the days of the afflicted are evil, But he who is of a merry heart has a continual feast.”

The NIV and GNT versions of verse 15 sound uncaring, they appears to say that victims of oppression or poverty will have to put up with being wretched all their days, while the lucky person (who is not oppressed or poor) can have a cheerful heart and a grand life. But in different translations, this verse has a variety of different meanings, and the KJV and NKJV use “afflicted”.

We can see the intended meaning of this verse by looking at the other verses in chapter 15. The general theme of the chapter is that:
(a) the quality of our lives, and
(b) the quality of the lives of those around us;
is not determined by external circumstances, but by what comes from within us – the personal choices we make, and the things we do and say in response to life’s challenges.

Verse 16 says, “Better a little with the fear of the Lord than great wealth with turmoil”
And verse 18, “A hot-tempered man stirs up dissension, but a patient man calms a quarrel.”
The New Living Translation reflects this theme in verse 15:
NLT “For the despondent, every day brings trouble; for the happy heart, life is a continual feast.”

This reminds me of the Catholic list of Seven Deadly Sins, the last of which has been called Sloth, but is better described as, “an unrelenting and wilful gloom and despondency, resulting in idleness.” Aquinas related this sin to bitterness, because those in its grips interiorized their anger, locking it up in their hearts and poisoning themselves. That view could lead us to take the KJV as “afflicting themselves”.

My personal thoughts on verse 15 are:
Within our limited human logic, life doesn’t make sense. Two summaries of typical human understanding I have seen were:
(1) a bumper sticker which read, “He who dies with the most toys wins”, and
(2) a work colleague who regularly exclaimed, “Life’s a bitch and then you’re dead.”
So, human understanding tends to lead to disappointment and despondency. And then sometimes bitterness toward others and toward ourselves, which is a certain cause of daily trouble.

But that is not what God intended for us. Dante described the seventh sin as the “failure to love God with all one’s heart, all one’s mind and all one’s soul.” When we do love God in that way, especially as we are aware of the sacrifice Jesus made for us on the cross, a natural consequence is “to love our neighbour as ourselves”, and to actively express God’s love in all we do. The transformed heart God creates in us will know true contentment in Him. It’s the perfect and only cure for despondency and idleness, as we daily enjoy God’s blessings, whatever our external circumstances.