After Luna's joy of bonding with the citizens of Ponyville Luna is initially ecstatic. However, she begins to feel resentful that there is only one night in which ponies go out and appreciate the night. Celestia grows worried that if something isn't done a repeat of Nightmare Moon will take place, and it will break her to lose her sister a second time. With Twilight's help the two of them make a plan to help Luna see she IS valuable.

Inspired by user SymbianL, who's picture of Celestia in the cover art directed me to a mournful song telling of Celestia's regret over sealing away her sister. The song inspired me to write of it. https://www.youtube.com/watch?v=5eSk0o8iHro