Many different answers have been offered to this question, and I think that the real answer is a composite of Biblical themes revolving around love, sacrifice, and fellowship. Certainly, God is self-sufficient, so there is no lack in Him--no void that must be filled. In 1 John 4:8 it says that God is love. In John 3:16 it says that "God so loved the world that He gave His only begotten Son . . . " We can see that the nature of love is to give. Furthermore, in John 15:13 Jesus said, "No greater love has anyone than that he lay his life down for a friend." In addition, 1 Cor. 1:9 states that we are called into fellowship with the Son of God, Jesus Christ.
Quite simply, it seems that God desired to create us in order that He might be able to love us. I think this is evident in the way creation was begun. God walked with Adam and Eve in the Garden. We see that after their sin, it was the Lord who came looking for them (Gen. 3). They are the ones who hid themselves. God sought them out. He desired to be with them and love them. Therefore, He set up the sacrificial system by covering Adam and Eve with animal skins. Of course, this was a representation of the sacrifice of Christ to come whereby Christians would be covered and redeemed. God's desire to fellowship and love us is not a weakness in Him but a manifestation of His character of love.
Love does not focus on itself but on others. God merely created the universe as a natural manifestation of His love. He populated it with people for whom He could express the greatest act of love, which is self-sacrifice. And thus He could give the greatest thing in the universe: fellowship with Him. In this, He is glorified.
If God had never created us, it would not lessen Him at all. He didn't need to create us due to any lack in Him. Rather, He merely desires to love us and fellowship with us.