There was only one explanation. For all of it. All of the strange, erratic behavior. Her irrational fear of the sun. Her nocturnal habits of locking herself in in the dark and closing all the blinds. Wearing her sunglasses everywhere, even inside! It was time to invest in some silver, garlic, and a wooden stake.

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.

Growing up is hard on its own, especially when your accomplishments are forever overshadowed by those of somepony else. And how could one not be overshadowed by Rainbow Dash?

Rainbow's son must learn to step out of his mother's shadow.

At the start of every storm, Pinkie Pie forsakes the safety and comfort of her home to gallop out into the rain and the lightning, and sit on the tallest hill in Ponyville. Everyone writes her behavior off as her "just being Pinkie Pie". That's true, though. She is being Pinkie Pie. Pinkie Pie would do anything to show a friend that she cares, and she knows one friend who is closer to the storms than anyone else. She knows that by showing her love for the storms when no one else will, she's showing her love for the pony who makes those storms.