[Author's note: This is a written version of the comics of My Little Pony: Friendship is Magic Annual (2013). I wrote this to get the lore of why Sunset Shimmer left Princess Celestia and this makes great storytelling as how Sunset Shimmer meets (Y/N)]
Narrator: It all began with Sunset Shimmer. She was born and raised in Canterlot as a Unicorn. She wasn't very close to her parents and never had real friends. She was educated at Princess Celestia's School For Gifted Unicorns around the same time as Moon Dancer, Lyra Heartstrings, Minuette, Twinkleshine, Lemon Hearts and a young Twilight Sparkle. She was a bright student with a future, but gradual went a dark path.
[Sunset Shimmer is at the examination hall at Princess Celestia's School for Gifted Unicorns. She is being examined by three Unicorn school instructors]
School Instructor #1: Now Sunset Shimmer... for your final test, you shall need to wow us. Give us the best you got.
School Instructor #2: We're looking forward to seeing if you can top your 'synchonized dancing cats' piece from last semester. That was a dazzler!
School Instructor #3: As Princess Celestia's own pupil, we expect big, big things today.
Sunset Shimmer: Well, you certainly see that [Pointing at the potted plant]
School Instructor #1: Oh! Looks like she's going to do a growth spell... classic! But is it enough?
School Instructor #2: Let's hope it turns out better than Juniper Berry's attempt.
Sunset Shimmer: [Whistling] [Uses a powerful spell to make the potted plant grow in massive size]
[Ponies cheers]
Sunset Shimmer: [Smiles] Thank you, thank you! It was nothing!
Princess Celestia: [Observing from a distance] I see somepony just passed their mid-term.
[Later that day]
[Sunset Shimmer was reading a book until she was interrupted while young filly Twilight, Shining Armor, and Cadence were busy spending time at the other side]
Minuette: Sunset Shimmer! That vine you made was amazing!
Twinkleshine: Really exquisite!
Lyra Heartstrings: Would you like to come have lunch with us?
Sunset Shimmer: [Closes her book and scoffs at the mares] I have better things to do than to SOCIALIZE. I'm going to re-read 'Advanced Elemental Magical Practices' and then practice for my advanced summoning spell exam. [Condescending] What are you studying for? Nothing, that's what. [Walks away]
[The girls were angry at Sunset Shimmer's remark]
[Meanwhile at the Canterlot Castle]
[Princess Celestia and Sunset Shimmer were walking down the hall]
Princess Celestia: Sunset Shimmer, I have heard that you're not getting along well with the other students. We've talked about the facts that personal relationships-
Sunset Shimmer: Personal relationships? Who needs anypony? The most important thing in life is to be the best.
Princess Celestia: We've talked about friendship and its importance before, and we've discussed that you need to start opening yourself up to new friends. And remember that talk about humility?
Sunset Shimmer: I remembered how boring it was.
[Princess Celestia showed her a room which she unlocked, where Sunset sees a mirror]
Sunset Shimmer: What is this?
Princess Celestia: Look into this mirror and tell me what you see.
Sunset Shimmer: [Rolls her eyes and then speaks highly of her beauty and her own self] A beautiful pony that has nothing but power and potential
Princess Celestia: [Sigh] [Rolls her eyes] Care to try again with our talk about humility taken into account?
Sunset Shimmer: [Holding in her anger] I see a pony who isn't powerful enough. Somepony who could be great, powerful...
Sunset Shimmer: Somepony that could rule Equestria.
[Sunset Shimmer saw something unusual as she sees herself in another form that wasn't a pony]
Princess Celestia: Maybe we better move on.
Sunset Shimmer: Wait, what was that? I thought I saw...
Princess Celestia: Oh, it was probably a trick of the light. Our lesson is over for today. For tomorrow, I want you to reflect on the discussion we've just had.
Sunset Shimmer: [Looks at the mirror] Hmm...
[Next day]
[Sunset Shimmer was reading a book in the library while Princess Celestia was organising books]
Sunset Shimmer: [Eager] About that mirror...
Princess Celestia: No. You aren't ready to know about that mirror.
[And the next day]
Sunset Shimmer: So... that mirror...
Princess Celestia: [Stern] No.
[And every day for week]
[Sunset Shimmer had a picnic with the Princess]
Sunset Shimmer: You know what's fascinating? Mirrors... about that one in-
Princess Celestia: [Losing her patience] Sunset Shimmer. We've been over this, we will get to the mirror and many other lessons in due time... when you are ready. [Walks away]
Sunset Shimmer: [Seething as she walks in another direction] [Mutters to herself] I am ready.
[Sunset Shimmer enters the Dark Magic library section while the librarian was sleeping]
Sunset Shimmer: Not ready? I'll show her not ready. [Noticed the sleeping librarian] You think this place would be better guarded.
[Author's note: Thank you, Sunset Shimmer! Somepony who made sense]
[Sunset Shimmer was rummaging through many books trying to find what she was looking for. The librarian wakes up to see Princess' Star pupil in the restricted area of the library as she quietly runs to report this to the Princess]
Sunset Shimmer: [Reading some book] Nothing on the mirror yet, but few of the spells will come in handy... [Reading Canterlot Castle A History: Volume One] Here we go! The crystal mirror... every thirty moons, a portal will open to another world?
??: [Angry] That is not really for you to know, now is it?
[Princess Celestia was behind Sunset Shimmer and the Princess has her guards with her]
Sunset Shimmer: [Looks behind her and was angry at Princess Celestia] How dare you keep this magic from me! You know that I'm ready for this, that I can be great!
Princess Celestia: You could be great. I thought I saw compassion and sincerity in you, but it was nothing but ambition. You're being selfish, you need to step back and reflect...
Sunset Shimmer: I'm selfish?? That book right there says I could become as powerful as an Alicorn princess. I could rule here. It's selfish of you to keep me from my rightful place.
Princess Celestia: [Angry]
Sunset Shimmer: I deserve to stand beside you and be your equal... if not better. Make me a Princess!
Princess Celestia: [Glares at Sunset Shimmer] No. Being a princess must be earnt... I have been trying to teach you everything you need to know, but you turned from it. Everytime you say you 'deserve' to get something without the effort just proves to me you are not ready.
Princess Celestia: [Stomp her hoof down] Sunset Shimmer, I am removing you from the position of my pupil. If we cannot get past this, your studies ends here. You are welcome to stay in Canterlot, but you are no longer welcomed in the castle.
Sunset Shimmer: [Seething with anger] We'll never get past this because you aren't seeing how great I deserve to be. Is that really all you have to say to me?
Princess Celestia: No. The guards will escort you out.
Sunset Shimmer: [Rage] This is the biggest mistake you'll make in your entire life.
[The guards escorted Sunset Shimmer out of the library]
Sunset Shimmer: [Looks at the mirror from the door and pretend to have fallen] It hurts.
Guard #1: Come on, Sunset.
Sunset Shimmer: [Headbutts Guard #1 and used her spell against Guard #2] Sayonara, suckers! [Enters the portal through the mirror]
[Sunset Shimmer enters a new world and she is a human]
Sunset Shimmer: [Looks at her body] This is... me! [Smiles] This is an interesting development. [Laughs]
[Meanwhile at the pony world]
Guard #1: [Moves the mirror near Princess Celestia's throne] Why hang it in an even more prominent place? Shouldn't you hide the mirror, Princess?
Princess Celestia: [Regret] No... I want it here... to remind me, also, maybe she will come back... someday. [Sad]
[End]