Why Sunset Shimmer Never Got Back Together With Flash Sentry · 6:33am Mar 20th, 2020
What I assume, based in part on the three photos of Sunset winning the Crown, is that the Sunset Shimmer who first came to the Humanoidverse was a much nicer person than the Sunset Shimmer we meet in the first Equestria Girls movie. Originally, she mostly manipulated others in what she considered was self-defense, considering that she had found herself in a society of (by Equestrian standards) cruel carnivorous ape-things.
When she got to know Flash Sentry, she was impressed by his many virtues (again, by Equestrian standards), intelligence, kindness and shared love of music. She gradually fell for him, and they came close to becoming lovers. (They mostly didn't because he didn't want to push her too hard, and she didn't want the sort of emotional bond that would have restrained her from returning home).
(As for Flash, though he does not realize this, he has an innate attraction toward Equestrian Unicorn mares. He's loved three girls in his life so far: Lyra, Sunset Shimmer, and Princess Twilight Sparkle. Lyra is soul-switched with her Equestrian analogue, and Sunset and Twilight ,actually are Equestrian mares polymorphed by the Portal).
Unfortunately, as Sunset Shimmer grew more secure in her position at Canterlot High School, she became increasingly cruel and amoral herself. She justified this by, essentially, anti-Humanoid racism -- since the Humanoids weren't quite Ponies, despite their intellects, and they were themselves crueller and more amoral than the Equestrian ideal, she was morally-justified in her manipulations by her superior civilization and need to defend herself against their predatory ways.
As Sunset Shimmer went from frightened to paranoid to amoral to increasingly evil, Flash Sentry noticed this transformation in her nature. He eventually could not continue to be a silent party her misdeeds. Flash tried to persuade her to better behavior, and when this failed, he broke up with her.
Though Flash could not persuade her, on some level Sunset still loved Flash, and was ashamed of what she was doing. This is clear from the fact that she never sought revenge on Flash, even when she had motive, means and opportunity in the period between the breakup and the appearance of Princess Twilight Sparkle.
Sunset started to realize the mistake she was making when she summoned the Shadow into herself and turned into the "She-Demon." This was because she was not fully in control of her actions as the monster. She didn't know exactly how much trouble she was potentially in -- she was duplicating the Fall of Princess Luna -- but she knew something was wrong. Look at her expression as she transforms.
When Princess Twilight and the Humane Five freed her from the Night Shadow, she knew that in the vain pursuit of a power she could not properly control, she had nearly given herself over to the dominion of a demonic force. What was more, she had given up the best chance she had ever had of real love, sacrificing it for an insane attempt to rule Equestria.
Sunset despised herself in that moment, especially when she saw the horror and contempt on Flash Sentry's face (look at his expression the first time he sees her after being freed from her mind control). She knew she had done much evil, and could not forgive herself.
She remains to some extent demoralized until she has to save her friends from the Dazzlings in Rainbow Rocks.
There are two big reasons she avoids trying to resume her romance with Flash Sentry.
The first is that she feels unworthy of him.
The second is that Sunset is still proud -- pride is a strong component of her nature -- and she resents that he broke up with her.
Unfortunately for them, Flash -- while nice, and even charismatic -- is neither a forceful personality nor one with much understanding of human (or for that matter Pony) nature. He doesn't realize that he needs to seek out Sunset when she's not trying to save the world, and emotionally force her into a serious conversation.
Perhaps he never will.
There's definitely a story here. One of Sunset's friends who does have a good understanding of human/equine nature—my money's on Rarity—telling Flash what he has to do, and what Sunset may need him to do. This blend of guilt and resentment may still churn in the depths of Sunset's heart. She's not one to let go of grudges easily, especially when there's shame entangled with them. Consider what it took for her to see Princess Celestia again.
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If I had to describe Sunset Shimmer's personality in two adjectives, I'd say "strong-willed" and "passionate." Which can cut both ways, depending on what she wants to do and why. Mostly, she's a good and honorable Pony now, but she hasn't always been.
Looking at that music video of his - which I parse as a dream - I kind of feel he also struggles with both being an unworthy outsider to her quite sufficient already social circle, and a certain degree of emasculation due to her superior powers and intellect. He really can't imagine what he can give her that she doesn't either already have herself or can't get better from one of her friends.
(Well, besides the obvious, but he prefers his relationships to based on emotional connections rather than just physical.)
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That explains why Flash Sentry has given up on trying to get back with her (remember, he's tried once or twice already), but it doesn't explain why she never sought him out again. I'm guessing mostly her pride.
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Given that he had clearly moved on to Twilight, I'm gussing she classified him as part of her past - which we kniw she prefers not to think about - and moved on. Really, the fact that he was her boyfriemd back then might not be a reccomending factor now.
In my fan ideas they get back together, heck I even have an anon a miss story fanwork idea where Flash stands by her side.
Great explanation, thanks for the info and headcanon!