Why is this?
The "taste" of eternity that life awakens in us fascinates us and draws us on, but it also brings a kind of sorrow because the fulfillment is not yet here, because we must wait. We must endure.
Nothing we do in this world can take this sorrow away, because we long for the Infinite—we really do! There really is a relationship between every event in our lives and eternity, and it is the secret behind every true joy. But even the joys of this life are permeated with the ache of longing, with the "not yet," as well as the apparently frustrating limits of our lives, which so often fall short of the mark of our destiny and bring us face to face with the obscurity of death.
In the end, only Jesus on the Cross makes this bearable: Jesus who makes the way through the pain of sin and death into that unconquerable joy to which each one of us is called by our Father.
Thus, in this life, even our deepest joy still has a note of longing. We share in the mystery of the Cross, and sometimes this "sorrow-in-the-midst-of-joy" seems sweeter than any regular satisfaction.
Other times it is as dark and lonely as death. Still the joy remains, as a (perhaps) impalpable but firm trust in God's promise. It hopes in the Resurrection.