I once met a man whose mood seemed unchangeable.
While most people I meet (myself included) have moods that are intimately bound to their circumstances, he seemed to move through his troubles and trials with ease.
When I tell of him, I am met with wholesale disbelief. If I insist, they follow with accusations that he must be unengaged with real life, or he is dangerously positive about everything, or has not had a significant problem. "No one," they contend, "can live like that all the time."
I believe he would respond to this with deep, heartfelt compassion, saying, "It is true that no one can have circumstances that provide it all the time. But to live like that all the time in spite of circumstances, yes they can."
I'm not saying he didn't cry, because he did, more often than most. And he got angry, though controlled, and for noble things. Distress invaded too, because he didn't seem to have a day without some sorrow, big and small, invading him or his line of sight.
But there is a difference in him. It's the difference between my millionaire friend needing to pay $1000 fine or go to jail, and my Filipino friend who makes that much in 8 months needing to. They are both good, and will pay, but their emotional response will be quite different.
He seems to walk atop a reservoir of joy that can handle life on its terms. It doesn't make him inhuman, just a more unshakable human. This, Paul says, is a fruit of life when it is lived in the Holy Spirit (Gal 5:22). And when it is gone in believers, Paul might well ask, "What has happened to all your joy?" (Gal 4:15)
Joy, I think, is the result of something good. Not a discipline. So the Spirit's untouchable joy comes when we really believe in an untouchable good news.