After her failure to be with Twilight, Celestia mopes around in misery over not having her one true love. Which is why Luna is set to find someone else who can be with her sister. Only thing is, Celestia didn't think him of all people would be the one Luna was looking for.
Big thanks for the help from Mr101 and Flint Sparks!
Coverart from the very talented Joey-Darkmeat.