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.
You should understand that we are at war and have been for a long time.
Our enemies foremost strategies have been to distract us and convince us that that’s not the case.
Therefore the first step towards victory is knowing in your heart that you are at war.