An excerpt from www.desiringgod.org:
John Piper explains that the command to renounce all means to abandon our pursuit of everlasting joy in earthly things. It is, as Jesus says in Matthew 13:44, our selling all we have in order to buy that field which possesses a treasure of infinite value.
“Renounce everything on earth,” Piper writes, “in order that you might have Jesus. . . . Jesus’s demand for self-denial is another way of calling us to radically pursue our deepest and most lasting joy” (85–86).
So rejoicing and renouncing are two sides of the same coin. If we are to rejoice in our heavenly hope — the fact that our reward is great in heaven — it must be because we ultimately have renounced our vain hopes in the things of this world. They are just toys, as C.S. Lewis reminds us, toys that were never intended to possess our hearts (The Problem of Pain, 107).
And therefore, we silence those false claims that the old person inside us would otherwise promote. We renounce them, and we set our eyes on heaven, even through the things of this earth, for behold, “[our] reward is great in heaven.”