From Joy and Sehnsucht: The Laughter and Longings of C. S. Lewis by Terry Lindvall:
What distinguished joy from happiness or pleasure was, for Lewis, a defining characteristic of longing—a deep yearning or poignant desire for something agonizingly elusive. Just as one’s pleasure in spring contains a memory of winter longings, joy for Lewis always contained "the stab, the pang, the inconsolable longing." This underlying quality of joy in Lewis’s system, then, was "that of an unsatisfied desire which is itself more desirable than any other satisfaction."
The German language has a word for this joyward longing that Lewis describes: sehnsucht. This is the haunting longing that touched Lewis throughout his life, that full, heavy, enveloping nostalgia for a fulfillment that awaited him—in something, somewhere.Read the entire article here.