Since her return and rescue, Princess Luna has been looking for some way to properly honor the six mares who saved her from herself, but nothing she has been able to think of has seemed meaningful enough to repay her saviors.
Nothing, at least, until she spends Nightmare Night in Ponyville, and gets to actually know the six ponies to whom she owes such a great debt. She realizes she enjoys their company, and wishes for them to spend more time with her, that she might grow closer to them.
So, in accordance with many fine and ancient traditions, and after thinking it over for a good...couple of hours, easy, she proclaims that she shall marry all six of them. Oh, and that Twilight will be marrying Celestia, too.
Now would be the appropriate time for everypony to panic.
--Cover Art Courtesy of Zemious--