In the beginning of the universe, there were thirteen spirits that created everything. Five were light spirits, that were responsible for everything good and light. Five were dark spirits, that were responsible for everything evil and dark. Three were neutral and served as the voice of reason amongst them.
When the Earth was created, a fight broke out amongst the spirits. The light spirits wanted the Earth to be a place of light and good. Always bright and pleasant to see at all times. The dark spirits wanted the Earth to be dark and a place for evil to reign.
The argument was taken to the three neutral spirits. One sided with the light, one with the dark. The final spirit saw both sides of the argument and could not decide. He thought long and hard about how to create a compromise between the dark and light.
After much consideration, he came up with the idea of a Sun for the planet. It would shine on the Earth, always lighting exactly one half of the planet. The lighted area would always be good, while the dark contained all the evil.
Eventually a war broke out between the inhabitants of the Earth along the border of the light and the dark. The light beings were too good, and the dark too evil. The spirits gathered to decide who should win. It all came down to the same deciding spirit, he needed to find a balance before the inhabitants of the planet destroyed each other. He decided to have the Sun circle the planet, it would still light exactly one half, but that half would always be moving. No people of the Earth would be good or evil all the time.
This is why the Sun rises and sets, to keep the good or evil from being better than the other. The Sun keeps the balance amongst our inhabitants.