Basically, the current theory is that Celestia is grooming Twilight Sparkle so that when her ancient wicked body finally gives out she can transfer her black soul into Twilight, and rule another thousand years.
"Equestria is mine. I am the Night and the Day. I am CELESTIA, I AM ETERNAL, AND ALL SHALL SERVE, BENEATH THE SHADOW OF MY WINGS!" - Celestia after slaying her mother and banishing her sister to the moon.