Here is a powerful saying on joy from what I feel to be Thomas Merton’s greatest work, New Seeds of Contemplation.
If I have [Christ’s] divine life in me, what do the accidents of pain and pleasure, hope and fear, joy and sorrow matter to me? They are not my life and they have little to do with it. Why should I fear anything that cannot rob me of God, and why should I desire anything that cannot give me possession of Him?
Exterior things come and go, but why should they disturb me? Why should joy excite me or sorrow cast me down, achievement delight me or failure depress me, life attract or death repel me if I live only in the Life that is within me by God’s gift?
Why should I worry about losing a bodily life that I must inevitably lose anyway, as long as I possess a spiritual life and identity that cannot be lost against my desire? Why should I fear to cease to be what I am not when I have already become something of what I am? Why should I go to great labor to possess satisfactions that cannot last an hour, and which bring misery after them, when I already own God in His eternity of joy?
It is the easiest thing in the world to possess this life and this joy; all you have to do is believe and love; and yet people waste their whole lives in appalling labor and difficulty and sacrifice to get things that make real life impossible.
This is one of the chief contradictions that sin has brought into our souls: we have to do violence to ourselves to keep from laboring uselessly for what is bitter and without joy, and we have to compel ourselves to take what is easy and full of happiness as though it were against our interests, because for us the line of least resistance leads in the way of greatest hardship and sometimes for us to do what is, in itself, most easy, can be the hardest thing in the world.