I wanted to make another upside-down Celestia picture but this time with Luna and thought this little goofy gag up.
So, after a thousand years Luna is once again happily home with her dear sister and everypony is happy to see her back in Equestria. Things have been going great so far...*cue Celestia walking on the ceiling*
Oh hello, sister!
... ... ... ... MIND = ASPLODE
And that's why we never see Luna in the series again.EDIT:
I added in the first panel to set up the joke. I think it works better this way.