From Derek Leman:
In Weight of Glory, Lewis calls this a “desire for our own far off country.” That may sound like a leap of logic, but it is a supposition. We “remain conscious of a desire which no natural happiness will satisfy.” And if we admit that this is true, suppose it is because we were made for another world. If we started with that supposition, we would expect to find longings for our true world in this one.The longing for bread, says Lewis, does not prove a particular man will be able to get bread. He may starve. But it does indicate that there exists somewhere a fulfillment for that need. We yearn for bread because we are made to need it. Our very yearning is evidence that something like bread exists. Whether we get any or not, bread is real.
Our desire for food and longing for romance would be strange if we lived in a world without food or sex, sustenance or family bonds. Our nature is a clue to reality. So why not believe the occasional flashes of intense longing for pure happiness, for uncontaminated beauty, for love uncorrupted are also clues that such a thing does exist?