Cloudy writes stories. Cloudy likes it when ponies like each other maybe a little too much and sometimes end up in love. Cloudy writes stories about these things, often.
Celestia likes her routine. Tax reforms, grants, laws and construction projects are all a princess needs to be content. It's just her luck that for Pinkie Pie, "content" is not nearly enough.
Ask Applejack, and she'll tell you Pinkie Pie can be a few apples short of a bushel. They've always been good friends, but what could they possibly have in common? Turns out there is an answer to that question.