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 Fluttershy finds out that Discord used to keep a mental diary while imprisoned in stone, she begs him to continue it. Grudgingly, he promises to document his thoughts about his new life among old enemies
Spike has had it. It's not his fault the library is a shambles. But he's not gonna take the blame. Until Twilight starts appreciating him, he goes on strike.
When a young pegasus with a gift for astronomy attempts to make a career out of his ability, he attracts the attention of several ponies, including the Princess of the Night herself...
After Celestia humiliates her at a dinner, Luna storms off and ends up snooping around in her sister's room. There, she stumbles upon the journal Celestia kept immediately after the banishment of Nightmare Moon.