I was born not alive nor dead, my world is full of the dead and the living. I was created to defeat time itself, born to love you so much you'll never ride it alone.
Without Christ, life is literally meaningless. This is why philosophers like Frederick Nietzsche and Albert Camus came up with complex philosophies to explain the emptiness of existence, and try to write their way out of the abyss in the process.
Ultimately, life is a Sisyphean struggle. But for the Christian, there is a transcendent purpose to the purposelessness. He is working for the next life. He is working towards Paradise. All his efforts, even if they seem vain, are actually deposits in the treasury of Heaven.
Plus, he has God working beside him. He has God to help with all his work. In fact, it is God who gives the Christian the superhuman strength to push the boulder back up the mountain again, especially after each and every fall. This mercy inspires us to keep going. And this is how one imagines Sisyphus is happy.