991 years. That's how long it's been since Nightmare Moon took over Luna and was banished. Every single year, Celestia has tried and failed to abate her pain over the incident. Finally, she has had enough of the hole in her heart and seeks to fill it, even if only temporarily. Becoming a mother to six seems like the best course of action to that end, doesn't it?
A little idea I had that I hadn't seen done before. Proofread and reviewed by Azure5555.