I'd honestly argue it's the consquences of Miyuki's neglect of Sorata up to this point.
Sorata's not a blameless victim, but he said this because he truly believes that he's getting in the way of Miyuki truly thriving. He's falling on his sword because he cares about Miyuki enough that he'd rather she reach her full potential, even at the cost of their relationship--which, again, he wants, but Miyuki always frames their engagement through the lens of their grandfathers' wishes.
And, Sorata's been getting the same sentiment of his inadequacy from everyone around them. That goes on long enough, and anyone would start to internalize it to the point they believe it themselves.
He certainly regrets saying it aloud, because doing so puts it out into the world where it risks being confirmed. But it's been on his mind since before even coming to Kyoto, and all his worries were coming to a head and tumbled out of him in this moment.
And instead of pushing back, Miyuki affirmed it, as Sorata always feared she would.
The reason Sorata is always looking for undertones in Miyuki's words is because Ayano screwed with his head when they were all younger, telling him that "[Kyoto Phrase A] means [Bad/Rude Thing B]"--out of anger at Sorata for coming in and "stealing Miyuki's potential away" because of something their grandfathers unilaterally decided.To add to this discussion, I really think that it just boils down to Sorata being around people from Kyoto too much that he tries to find hidden undertones in what they say instead of taking their statements as is. And Miyuki, being from Kyoto, says or does things naturally that Sorata thinks was in "Kyoto speak" instead of being literal. I really think that this is a great way of using the "misunderstanding" trope to create tension.
👆Bro, how did you think she was going to react?
👆
"I know you just told me you chose to be with me of your own will, and that you chose the school makes you happy, but I'm going to say you should do the exact opposite of what you said you want. Because your brother and I know you can do better than that."
…
"Wait, was that bad?"
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Before you assert that I don't know what I'm talking about, i.e. "You should maybe go back and read the actual dialogue":You should maybe go back and read the actual dialogue of the chapter.
All Miyuki says is, quote,
"Coming to Tokyo was my choice."
"Adding onto that, I interpreted Grandpa's words as, 'go to the same school as Sorata-san'."
No statement of choosing Tokyo because it makes her happy. No statement of intent on going to be with Sorata.
Not once did she say she wants to be Sorata of her own will.
If he actually understood that she's choosing their school to be with him of her free will, Sorata would would also have accepted that she has special for him.Before you assert that I don't know what I'm talking about, i.e. "You should maybe go back and read the actual dialogue":
Why are you deliberately ignoring the fact "Going to Tokyo was my choice" fits exactly what I said, that it was her choice?
And the fact that she follows up by filling in the rest of Ayano's position with "and that… going to the school that fit me was a waste"?
She's contradicting it by saying "No, the school I'm going to is the one that fit me" and making clear that she's not happy that Ayano is saying it's a waste. And she's already stated previously that she's happy going there, whether he remembers or not. Again, he's saying that he agrees with her brother that even if she chose otherwise, he and her brother know what she needs better than she does.