God didn't need to make the world. He created the universe because he wanted to.
Perhaps it is not surprising that God did decide to create the universe. God has revealed to us that love is a fundamental description of who he is. And if God is love, it is perhaps not surprising that he would create more things to love. 
But, admittedly, it is highly doubtful that we could comprehend God's literal motivations for creation. We inevitably "anthropomorphize" when we think of God. We create him in our image. We ascribe feelings and motivations to which we can relate.
Of course God has, in effect, encouraged us to do so. After all, that is the way he has revealed himself to us in the Bible. To varying degrees, the Bible is full of anthropomorphisms. For example, an all-knowing God cannot literally regret that he made humanity, nor will he be deeply troubled literally (Gen. 6:6). He knew humanity was going to do such things, and he knew how he was going to respond. He knew the world would become vastly wicked and that he would destroy them with a Flood.
Such statements were God meeting Israel where they were, God revealing himself in terms to which we can relate as humans. Such language is "incarnated" revelation. It was God taking on human flesh in terms of how Israel and the original audiences of the Bible understood things. Such biblical language shows us what God is like by analogy to our human experiences, but it is not precise or exactly literal language.
So what pictures does Scripture give us of God's motivations for creation? Revelation 4:11 simply mentions that everything was created because of God's will. In other words, God created the universe because he wanted to. The context of this verse is the final restoration of the universe to its ideal state, with all creation worshiping its Creator.
In Acts 17:25, Paul makes it clear to his pagan Greek audience that God does not need to be served with human hands, like the gods of Athens that Acts was critiquing. Rather, God himself is the source of all life. God himself made the world and everything in it. He does not live in temples made by human hands.
As the sermon we call Hebrews makes clear, the temple of the Old Testament was only meant to point to God in a shadowy way. It foreshadowed the perfect sacrifice of Jesus on the cross and, like all revelation, gave a picture of God that Israel could understand in terms similar to the way all the people of that day worshiped their gods. But God does not literally dwell in houses made by human hands (cf. Acts 7:48-50.
Even Solomon, when he dedicates the temple in 1 Kings 8:27, recognizes that "The heavens, even the highest heaven, cannot contain you." Solomon himself, in the paradigm of his day, probably pictured the heavens as layers of sky as you move upward until you reached the highest heaven where Yahweh dwelt. But we can translate his picture and see that this universe cannot possibly contain God. Indeed, as Christians we believe that God created the universe out of nothing and thus that God in his essence is distinct from this universe.
Before God created this universe, Father, Son, and Holy Spirit alone existed and were completely self-sufficient. Christian tradition calls this characteristic of God his "aseity," the fact that he does not need the universe. We should not take that to mean he is aloof or disinterested. It is to say that while we could not exist without him, he does and could exist without us.
And he would not be God if it were not the case. The gods of the Greeks, Babylonians, and all the other ancients were not nearly so powerful as the God we serve. Indeed, they were really only larger than life versions of the men and women who imagined them. They threw temper tantrums, had affairs on their spouses, and could be defeated in battle. They were really just projections of really powerful men and women who could not die.
None of these descriptions apply to the one true God. Even if the Bible might use anthropomorphisms like God getting angry or changing his mind, these are just pictures to help us understand. They are baby talk meant to help us catch a glimpse of a God whose ways are "past finding out" (Rom. 11:33, KJV). We can catch a glimpse of what it might mean for God to have all power, but only a glimpse. We can see by analogy to things we know just a small taste of what it must be like for him to know all things.
But the literal essence of God, what he is apart from this creation, is beyond anything we could imagine (Eph. 3:20). We know him primarily in relation to the creation of which we are a part. To think we have a grasp on him literally--as he actually is without figure or metaphor--runs the risk of idolatry, of mistaking an image for the Creator himself.
While Christians disagree on many of the nuances of God and theology, faith in the self-sufficiency of God has been nearly universal throughout Christian history. There might be some today, particularly those who identify as process theologians, who might question the self-sufficiency of God. Process theology tends not to see God as distinct from the world but developing and becoming with the world. But this approach to God deconstructs the very notion of God into something else.
To say that God is God is to say that he does not need anything outside himself. God didn't need to create the world because he was lonely or because there was something imperfect about him without us. God created the world because he wanted to.
Next Sunday: G2. Is God a guy?
 Christianity holds that the one God mysteriously exists as three persons--Father, Son, and Holy Spirit. Thus, Christianity would traditionally believe that the love within this "Trinity" was a sufficient expression of love for God. He did not have to create more because he is love. The love of the Trinity was sufficient.