This question has puzzled theists for millenia, and its atheistic equivalent, "Why is there something rather than nothing?" has puzzled everyone else for equally as long.
Augustine says, "But why did God choose then to create the heavens and earth which up to that time He had not made? If they who put this question wish to make out that the world is eternal and without beginning, and that consequently it has not been made by God, they are strangely deceived, and rave in the incurable madness of impiety. For, though the voices of the prophets were silent, the world itself, by its well-ordered changes and movements, and by the fair appearance of all visible things, bears a testimony of its own, both that it has been created, and also that it could not have been created save by God, whose greatness and beauty are unutterable and invisible." Augustine is here concerned to demonstrate that the world is not eternal, but created. The question remains, if it was indeed created, then why?
Plato's Timaeus describes a universe before creation in which the only two things mentioned are the divine creator and the "models" or paradigmata that the creator uses as a reference when he creates. The models are real, ideal forms, and the creator is a mysterious character who ought not to be confused with the Judeo Christian God. Timaeus describes the divine creator's motivation, "He was good, and one who is good can never become jealous of anything. And so, being free of jealousy, he wanted everything to become as much like himself as was possible." But the model used by the creator the in creating the physical world is therefore prior (ontologically, if not temporally) to the creator himself. Timaeus does not describe there is "something other than the form of the good, rather than nothing." The question for Timeaus then is once removed, "Why is there a divine creator rather than simply the ideal formal reality and nothing else?"
And in the East, the question of the world's eternality or temporality is considered by Buddhists to be one of the "unanswerables," one of the "questions which tend not to edification."
Here is one poet's attempt at answering the question by a "back-door" strategy. He takes for granted the futility of our rationality's ability to reach the end of the inquiry, and instead seeks knowledge by direct communication with the creator. The poem, a story from the perspective of King David, is by Jalaludin Rumi, from the book"Love is a Stranger", Kabir Helminski, Threshold Books, 1993:
"Lord, said David, since you do not need us,
why did you create these two worlds?
Reality replied: O prisoner of time, I was a secret treasure of kindness and generosity, and I wished this treasure to be known, so I created a mirror: its shining face, the heart; its darkened back, the world; The back would please you if you've never seen the face.
Has anyone ever produced a mirror out of mud and straw? Yet clean away the mud and straw, and a mirror might be revealed.
Until the juice ferments a while in the cask, it isn't wine. If you wish your heart to be bright, you must do a little work.
My King addressed the soul of my flesh: You return just as you left. Where are the traces of my gifts?
We know that alchemy transforms copper into gold. This Sun doesn't want a crown or robe from God's grace. He is a hat to a hundred bald men, a covering for ten who were naked.
Jesus sat humbly on the back of an ass, my child! How could a zephyr ride an ass? Spirit, find your way, in seeking lowness like a stream. Reason, tread the path of selflessness into eternity.
Remember God so much that you are forgotten. Let the caller and the called disappear; be lost in the Call."