So, I only just started watching MLP:FiM like a month ago, but I already have a (tentative) headcanon for this.
Most ponies of modern Equestria know of Princess Celestia as an wise-but-aloof benevolent monarch, full of royal dignity and waited on hand and foot by the ancient Unicorn aristocracy of Canterlot Castle. But it was not always so...
A thousand years ago, "Princess" Celestia was a wildly hedonistic party girl slash sun goddess who hosted the most terrifyingly extreme ragers you could imagine in the middle of the Everfree Forest. Not a single one of these parties ever managed to end before every partygoer was seriously threatened by some kind of monster or out-of-control spell. Celestia of the Day had all the enthusiasm of Pinkie Pie, the recklessness of Rainbow Dash, and the magical prowess of Twilight Sparkle, and she was having an absolute BLAST with it.
But, as explained in the very first two minutes of the show, Luna grew jealous of the way ponies "relished and played in the day [Celestia] brought forth", turned into Nightmare Moon, got exiled to the moon, yada yada yada you know the story. But the point is, now not only did Celestia have to take sole responsibility for both the day AND the night, she also had to pick up and move all the way over to Canterlot Castle, because her & her sister's sweet custom-built party palace in the woods was now completely uninhabitable. And you know who was already living in Canterlot? Nobles. All the snootiest, most uptight, unicorn nobles in the land were all demanding Celestia's attention 24/7 at the same time that she suddenly had to handle a whole new domain being added to her godly duties.
Frankly, it's a testimony to the Princess' deeply embedded sense of duty and integrity that she didn't just immediately flip out and raze the city to ground.
But NO, she had to be a GOOD ruler. For the sake of the kingdom, she could replace her epic ragers with charming little soireés and grand, dignified, balls. She could replace her adrenaline-fueled monster battles with scintillating party conversation. She could dispense her divine wisdom while seated on a dignified throne instead of sprawled out on the sun-scorched grass. She just had to suffer through the mind-numbing tedium of it all for an entire. Freaking. Millennium.
But eventually, a ray of hope appears! A little unicorn filly appears, reversing gravity, turning people into potted plants, and aging baby dragons into full-grown giant monsters! It's just like Celestia's old party tricks form back when she was allowed personal expression! Immediately, she rushes to the scene and declares this young filly to be her new protegé! After all, even if Celestia herself can't have fun anymore, surely nobody could object to her living vicariously through another, officially non-royal, magical party girl!
...And then, in their very first lesson together, it becomes abundantly clear that this "Twilight Sparkle" is actually the biggest, most boring, loser nerd in existence. She stubbornly resists any and all teaching methods that don't involve lectures and assigned reading. She cries if Celestia tries to do anything with her that wasn't pre-planned on a neatly formatted checklist. Even the simplest, least formalized versions of socializing send her into full-blown panic attacks. It's like the universe is MOCKING Princess Celestia.
At one point, she's suffering through yet another round of preparing for the unicorns' fancy-schmancy Summer Sun Celebration, when Twilight sends her yet another tiresome book report of some kind, which she only briefly skims because it is frankly. Just. One. Book report. TOO MANY. Fully at her wit's end by now, she sends Twilight off to Ponyville, the nearest remaining city to her beloved old palace, and tells her to not come back until she's gotten a damn life, learned how to party for real, and made some goddamn friends (except, you know - said in a regal, benevolent, way).
And so then, well...the events of the first two episodes of the show happen. Turns out Celestia was so exhausted from all that mind-numbing tedium of being a Proper Canterlot Queen, she completely forgot that she had arranged for Luna to be let out of her prison this year. Turns out that's what Twilight's last report was about all along, haha whoops. But! It all turns out ok! BETTER than ok, actually, since against all odds, Twilight actually DID make some friends in Ponyville, AND they managed to scrounge up the Elements of Harmony from that room in the old palace where Celestia had left them! So Celestia desperately manages to fake her way through a quick speech to Twilight about how this was her plan all along, I knew you had it in you, yada yada yada oh my God lil' sis welcome back Imissedyousomuch PLEASE take your old job back pleasepleaseplease-
So anyway, after this point, Celestia just kinda does whatever she can think of to turn Twilight into someone she can actually live vicariously through. Granted, she has to do it pretty much all through letters, since even with Luna finally back to handle her nighttime duties she still can't seem to get away from her royal minders at Canterlot, but at least once in a while she gets to see her little student & her friends wreak havoc in person, so it's not so bad as all that.
