Back again.
So, according to “City of God”–
The universe was created because God is infinite. God is perfectly aware of his own infinite attributes and perfections. Likewise, he has an inclination to communicate himself outwardly. So it naturally follows that an infinite being would want to give himself the opportunity to infinitely share himself and all that he possesses. And rather than diminishing his own gifts and graces, it merely serves to further increase them by giving himself an outlet for something that’s already inexhaustible. God is ready to overwhelm all of creation with his love and gifts, and the only thing that can hold that back is sin.
Regarding humanity, I thought about paraphrasing but decided to just quote part of it. “[Next] was decreed the creation of a people and congregation of men for Christ, who was already formed in the divine mind and will, and according to whose image and likeness man was to be made, in order, that the incarnate Word might find bretheren, similar but inferior to Himself and a people of his own nature, of whom He might be the Head.”
Anywhoo… it seems reasonable enough to imagine that a God who had infinite love, and infinite graces, and infinite treasure would want to create something to channel that love, channel those graces, and channel that treasure so that he wouldn’t be the only one who benefited from it. When you’ve got something fabulous, you want to share it with others… God just has the option of actually creating others to share it with.

And likewise, you like others to know where, exactly, those fabulous things come from. The Incarnation would have occurred whether there was Original Sin to save us from or not, because Christ not only redeemed us through his sufferings, but also used his ministry to bring us closer to God and give us a clearer idea of God’s expectations of us. So man was created not only to benefit from God’s generosity, but also to be a people for Christ.
But it also follows— if we were created to benefit from God’s generosity, how well have we used the gifts we’ve received? Are we the wheat on fertile ground that gives back a hundredfold, or have we given God interest on his talents of silver, or are we good and fruitful trees in his vineyard? If we were created to be a people for Christ, how well are we doing in our efforts to be Christlike? In such circumstances, what are our responsibilities and obligations to God? And to our fellow man?
Good food for thought.