Ralph C. Wood is commenting here on The Lord of the Rings, but this is also a pretty good description of the quintessential Christian demeanor in a post-Fall, pre-glory, already/not-yet age:

Treebeard possesses what might well be called the essential Tolkienian demeanor—a fundamental somberness about the world’s state, yet with an overriding joy that cannot be quenched:

“Pippin could see a sad look in his eyes, sad but not unhappy ([2.90; my emphasis).

The Gospel According to Tolkien: Visions of the Kingdom in Middle-earth (Louisvile, WJK: 2003), 18.

The Apostle Paul put it this way:

“We are . . . sorrowful, yet always rejoicing.”

—2 Cor. 6:10

Update: Tim Keller’s comment below is helpful:

This is a very important theme in Tolkien. The elves are often described as both old and young, both joyful and sad. A more explicit expression of it is the description of Gandalf in Book 3–

. . . in the wizard’s face he saw at first only lines of care and sorrow; though as he looked more intently he perceived that under all there was a great joy: a fountain of mirth enough to set a kingdom laughing, were it to gush forth.

And I agree—it is very helpful in describing the demeanor of Christians, who will feel the fallenness of the world most keenly because they know what God created the world to be, and who know that nothing in history will ever bring about any fundamental repair of things, and yet Christians also have an unquenchable, infallible assurance that in the end, everything will be joy and glory. So how else can we act, but “sad, but not unhappy,” “afflicted, but not crushed”—weeping, but rejoicing.