Sehnsucht (why no one is happy)

It is part of the human condition to be unsatisfied with our lives, no matter how rich, famous, or powerful we are. No matter what we do, we will always long for something more to satisfy the longing we have for we know not what. This longing inside us is called Sehnsucht. The desire for the fulfillment of Sehnsucht is part of what it is to be human. We all want it, just as much as we want to breathe air or drink water. For this reason, we call Sehnsucht a natural desire.

We find in the world a fulfillment for all of our natural desires. We hunger, and there is food. We tire, and there is sleep. How strange it is then that we do not find a fulfillment to Sehnsucht in this world. But as a natural desire, it must have some real fulfillment, else it would not be part of our human nature (nature makes nothing in vain). Therefore, it must be that the fulfillment to Sehnsucht is not in this life, but the next. This is what people call Heaven, or life with God forever.

CS Lewis has made this line of reasoning popular: "If I find in myself a desire which no experience in this world can satisfy, the most probable explanation is that I was made for another world."

You can read a longer explanation of the argument from Peter Kreeft here: https://www.peterkreeft.com/topics/desire.htm