Why would an eternally happy God create the universe? Without the Creation Jesus would never have had to suffer. The Father would not have had to turn away from his own beloved Son. The Spirit would never have been blasphemed and offended. They could have been eternally happy together without us!
In my book, Covenant and Community, I take some space to emphasize that the central motive (from the Father's perspective) behind the creation of the universe, creation of humanity, human history, the sending of the Son to Earth, the crucifiction and resurection, and the marriage supper of the lamb~the central motive of it all is the Father's love for his Son. This whole grand creation is a means whereby the Father can bring glory to his Son, shower his love on his Son, elevate his Son. I hesitate to give citations for this doctrine, because that makes us think that we only believe what is stated catagorically, whereas this fact is attested to throughout the New Testament as a constant theme that is being developed but not always explicitly stated. But if you insist on catagorical statements, look at the first chapter of Ephesians, the middle portion of John 5, the first few and last few chapters of Revelation.
Jesus insisted that his own actions, both those that made the crowds love him and those that lead to his death, were proof that the Father loved him. You may answer, "For God so loved the WORLD. . .." Yes, that is true. Jesus came into the world because of the Father's love for the world. But why was there a world to love? What motive was there to create in the first place? I think that the consistent thrust of Scripture is that the motive for Creation is "For the Father so loved the Son. . .." That love was prior to and always has priority over his love for us.
Consider Jesus' words, "If anyone loves me, he will obey my teaching. My Father will love him, and we will come to him and make our home with him." The Father loves us for the Son's sake. He loves the one who loves his Son.
If this is the starting point for God, should it not also be our starting point? Should not the Father's love for the Son be at the very center of our understanding of what the Gospel is? Should it not even be the starting point in our evangelism? Jesus repeatedly said that the things he did and the things he said were so that people would know that the Father sent him, hears him, loves him. He wanted us to see their relationship.
(This is a very truncated explanation. In its full scope it should acknowlege that the Father's love for the Son is fully reciprocated and the Son is using the Creation and his own incarnation to heap up honor to his Father. Also, their relationships with the Holy Spirit display more intricacies of loving communion. This is gone into in more detail in Covenant and Community.)
Should not all of our faith, doctrine and conduct begin with the relationships of the Father, Son and Spirit, for it is those relationships that prompted even our very existence.