Tolkien believed that creating, especially fantasy, was one of the most natural human activities. He believed that we make and are motivated to make because we are made. Not only have we been made but made in the likeness of a Maker; meaning, as creatures, we are given certain characteristics of the Creator, one of which is His creativity. He believed that when humans create (art, story, language, music) they do so as sub-creators reflecting the activity of the Primary Creator, God.
It is unmistakable the influence that Tolkien's Christian faith had not only in his story but also on his motivation to create worlds through story. Tolkien had a worldview where he could make sense out of the need to create, and justify his purpose in making. Because Tolkien believed that he was made in the image of God, he made every effort to create in a way that acknowledged and glorified the One who created him. His sub-creation shines with the Imago Dei. Indeed, it is my conviction that any worthwhile sub-creation, whether it be the visual arts, music, story etc., does this even if its author didn't intend it to. In fact, many times it shines in spite of its author's intentions. Whatever we make is derivative of the One who first made, therefore its value comes from how well our sub-creations represent, capture and reflect the image/characteristics of the Creator. All creations that are objectively valuable bare the Imago Dei to some degree; some more than others. It is not surprising, therefore, given Tolkien's worldview, that his art is stamped all over with the image if his Creator.
In an attempt to further explore the question of why Tolkien expended such great effort on his fiction to give it a sense of reality, there is another area to explore. That is fiction's ability to relate to reality and how this could help us understand what motivated Tolkien. The plan is to explore this topic more in depth in yet another post.