Back in the early days of Equestria, when Luna was still with her sister and Discord was still flesh and blood, a young pony discovers that she is bored with her charmed life. Her name was Celestia, although she prefers to go by Sunny Days.
You see, Celestia feels a bit pressured by her newly approaching 'royal duties,' so she created a new personality for herself: mild-mannered filly-next-door Sunny Days.
Of course, she didn't account for the odd little creature that would soon become her biggest fan. His name was Discord, and he was quite possibly the most annoying and obnoxious animal on the face of the planet.
So love was a foregone conclusion.