Description
Celestia needs more magically power to keep discord in check. Dragons make ponies alicorns and supply the magic to their pony parents as soon as they hatch. Her dragon was always drained, so she planned to have an unlimited supply. That's where her young pupils came in, twilight and cadance, who both were powerful enough to break a dragon's egg. Too young to tell what was going on they comply with their queen's wishes of draining their dragon's magic for her.
Now that discord has calmed down and twilight is now older, she isn't close to her old teacher for using her like that. As a princess, she can't escape the sun and finds she spends time keeping up with the school of magic, raising spike, and hanging out with her friends without the sun's input. Spike and her have become the same person as he tries to emulate everything she does.