Lovers have their reasons. To Twilight Sparkle, the reasons she loves Celestia are perfectly obvious. And to Princess Celestia the reasons why she can't love Twilight back are equally obvious.
When Twilight visits Celestia's private hall of stained glass windows, the portrait of an ancient astronomer catches her eye. Who was this pony? Why was she so important to Celestia? And why does she look so familiar?
Celestia and Twilight are in a relationship. Twilight wants to meet Celestia's parents. Celestia, understandably, is terrified. After all, it's always awkward when one's parents aren't exactly ponies at all.
My sister is an idiot. Her faithful student, Twilight Sparkle, appears to share her stupidity. Luckily, I am here to lend them a helping hoof. I can't /really/ blame them, though. They're so /adorable/ when they're clueless.
During what started as a perfectly fine Lunch, Twilight learns that her friends have somehow gotten it into their heads that she is in a relationship with her mentor and idol Princess Celestia.
After spending Nightmare Night in Ponyville, Luna decides on how to reward the six mares who defeated Nightmare Moon and saved her from herself: Marriage.
Celestia is struggling to come to terms with her true feelings for Twilight. Unwilling to watch her sister suffer under apathy and indecision, Luna takes it upon herself to help Celestia confront her newfound feelings.
Time traveling from five thousand years in the future, Twilight Sparkle—or as she prefers, Glitter-Flanks—has one mission: to establish the closed time loop that sets her and Celestia up as a couple, by flirting as aggressively as possible.
Celestia has seen a lot of terrifying things, but nothing she has ever faced is more terrifying to her than this: Telling Twilight's parents she's in love with their daughter.
Twilight panics when she discovers that Princesses are expected to have formal dates for Hearts and Hooves Day, so she travels to Celestia for advice, getting a lot more than either of them bargained for in the process.
Twilight has agreed to host Princess Celestia for a three month vacation. When she meets her disguised form, she sees her teacher in a whole new light.