The New Earth is going to be so amazing, wonderful, beautiful, breath-taking, tangible, relational, joy-filled (the list goes on and on) and is also void of pain, suffering, death, sin, and grief. In light of all that, how much will rewards really matter?
First of all, rewards matter first and foremost because they matter to God. It should matter precisely to us because it matters to Him. The New Testament is full of references to rewards. The differences in position of leadership will be significant. The joy of having more of God will mean that some people’s capacity for joy will be greater (i.e. the Jonathan Edwards’s analogy, vessels are all full, but larger vessels have more).
It’s like saying “I don’t understand how we can learn more if we already know a lot.” The base line is a huge amount of knowledge but it’s not an infinite amount of knowledge. There is joy beyond imagination but not infinite joy because we aren’t infinite beings. If knowledge can increase, why can’t joy?
Think of Heaven as a place of abundant happiness but not infinite happiness. Nothing in our lives can be infinite because we are finite.
For more information on the subject of Heaven, see Randy Alcorn’s book Heaven.