Humans are constantly seeking meaning and purpose in their lives; they want to feel that they matter in the grand scheme. Atheists have come to terms with the fact that there is no ultimate purpose and, so, assign purpose to their own lives.
But let's imagine God is real, and that everyone's life is validated by his/her existence. This god has a plan for each person's life and, when they get to Heaven, God reveals just how vital their role was in his Plan. All these people feel fulfilled and content that they mattered. They are not just debris floating in the deep, dark, lonely space.
Now let's imagine being God. You're all alone in space, drifting through time with no purpose. You have no one to talk to; no one who cares you exist. What do you do? You have a lot of time on your hands after all, and so you create human beings (and other animals) whose sole purpose is to acknowledge that you exist and that you matter. Without the existence of these creations, no one knows you; you are just debris floating in the deep, dark, lonely space.
God is just as irrelevant without his creation as humans beings feel they are without a Creator. Without us, he/she has no purpose. This is much like the question that is always begged when Creationists say that nothing cannot come from nothing, yet somehow God can come from nothing. If human beings can have no purpose outside of a Creator, how can a Creator have purpose outside of a Creator? If there is no Higher Intelligence to acknowledge even the existence of God, how is his/her existence meaningful in the grand scheme of eternity? It's not.
What is more pathetic than the striving of human beings to prove their superiority to other floating debris is the striving of a god to prove its superiority to other floating debris. No matter how far out you go, nothing can validate the life of any one being but that being itself. No matter how much praise this imaginary god gleans from his creation, his/her life is just as meaningless as the rest of ours...
...unless we claim our own lives. "I think; therefore, I am." I am; therefore, I live. I live; therefore I will live... and love, and learn, and make the most of it while I am alive.