You remember the Mare in the Moon story we're told at the very start of the show? Well, I have a theory that will expose it as a lie...
What were we told from the story? Celestia and Luna were 2 sisters that raised and lowered the Sun and Moon, but Luna became jealous because she wasn't getting any attention and decided to not lower the Moon one night. After that, she became Nightmare Moon, and Celestia reluctantly used the Elements of Harmony to banish Nightmare Moon on the Moon for 1000 years, and the show starts one the 1000th year.
I don't think this is actually true. No, I believe Celestia told everypony this story...in reverse.
If you pay attention to Celestia's behavior and mentality while interacting with Luna throughout the series, she seems to have a know-it-all and "I come first" attitude. It implies that Celestia pushes Luna aside while she gets the spotlight. If you take this and apply it with my theory that Celestia is using the concept of friendship to brainwash and indoctrinate everypony to expand her kingdom, then you can see that Celestia is pretty arrogant, manipulative, and controlling. So, when you look at the Mare in the Moon story from Luna's point of view, all she wanted was some of the attention and some credit, but the ponies did their thing during the day while sleeping during the night. It's clear that Luna felt left out and unappreciated. Remember when the story said that Luna wanted the night to last forever and turned evil when she refused to listen to Celestia? The story is making Luna look like a spoiled brat for not getting her way. Doesn't it seem like the story is trying to make Luna look like the bad guy while glorifying Celetsia at the same time? I think Celestia herself wrote the Mare in the Moon story, and she did it in a way that will make her look good and Luna bad. This gives off the impression to the story's readers that Celestia is the superior princess and the one you need to trust, and this, in turn, will make everypony discredit and disregard Luna.
I could leave this here, but I think there's more to this. What if the Luna we've seen isn't the real Luna? What if...the original Luna was murdered by Celestia? "Oh, buck off, there's no way that Celestia would do something like this! " you Celestia fans out there, but just hear me out, here.
When Luna refused to lower the Moon, the story claimed she was being a brat, but she was trying to stand up for herself from Celestia's mental abuse towards her. After Luna transformed herself into Nightmare Moon, Celestia realized that she wasn't gonna stay submissive and obedient to her authority anymore, so she decided to murder Luna to make sure her authority and rule wouldn't be challenged or overthrown. After the murder, Celestia told everypony that Luna went rogue, but this was a story she used to gain the trust and support from her subjects. So, Celestia basically covered up Luna's murder in order to protect her reputation and to keep her power. Celestia knew that somepony could eventually figure out the conspiracy, so she decided to make a clone of Luna as a fail-safe so she wouldn't be found out. When the events of the first 2 episodes took place, it was time for Celestia to fake Nightmare Moon's return. Remember when Celestia seem to disappear and Nightmare Moon appeared? I believe that the Nightmare Moon we see in those first 2 episodes was really Celestia in disguise. This explains why Celestia was missing without any trace or explanation. When Nightmare Moon was defeated, Celestia put the Luna clone in her place, and she reappeared, making it look like she was released from where Nightmare Moon supposedly banished her to. The idea that Nightmare Moon was Celestia in disguise makes sense because, after Nightmare Moon was defeated, she appeared out of nowhere. Again, we're giving NO explanation for how Celestia supposedly disappeared by Nightmare Moon and where she was. It makes all of this seem to be a set up. And did you ever notice how easily Nightmare Moon was defeated? I mean, she didn't even put up a fight. And all the damage she made was easily reversible.
Now, where does the "told on reverse" bit come in? Celestia didn't want competition from her sister when it came to ruling Equestria. Luna was the one who was trying to reason with Celestia about sharing the rule 50-50, but Celestia wasn't about to give any of it up. This is what the "Luna refused to lower the Moon to make room for the day" bit was really; it was a metaphor of one not sharing the power with the co-ruler and keeping it to themselves. The Mare in the Moon story shifted this aspect onto Luna, to make her look like the corrupted one.
The Luna clone (the Luna we see in the show) was given the false memory that was the Mare in the Moon story by Celestia to make sure that the lie wouldn't be exposed, and her agenda would continue to be fulfilled without interruption...
What do you think of this theory?