There are only two reasons anyone ever does anything: because they have to, or because they want to.
That means that God either wanted to create humans, or had to create humans. We must be the result either of God's desire, or of God's need.
I'm voting it's God's desire. Because how could God have needs? He's full unto himself. Safe to say, God, being absolute, is absolutely satisfied. God doesn't have needs.
That means we're the result of a desire God had. He chose to create us.
But why would God make that choice? What's in it for him? What desire of God's are we satisfying just by being alive?
Oh, sure, he gets 10% of our money. But c'mon. It's not like God's up there sitting at his kitchen table clipping coupons. I'm sure he has someone do that for him. (Okay, on a side note: Can you imagine being an angel, trying to buy a gift for God? Talk about the guy who has everything.)
It's hard to think of why a perfect, perfectly fulfilled being would want to create humans. What would motivate God to go, "You know what I feel like doing? Creating a race of beings who walk upright, are consciously aware of themselves as separate from their surroundings, and who sense but cannot objectively varify my existence. The monkeys have been fun, but these'll be even better. Upright, less hairy, infinitely smarter monkeys! Sweet!"?
So here we are.
What's in it for God?