It's funny how some people ignore Miyamura's more sinister sides when shitting on this story (not responding to anything here, just in general when I've seen this story come up, either anime or otherwise)- so many people be like 'why Hori so mean to this cinnamon roll'. I mean, yeah, a lot of the time, that's the side he presents because he's pretty gentle... but Hori's shown a lot of caring, as well, especially with her little brother.
But, then you have Miyamura's death glares and blows he's thrown at Tooru, if only for that one moment before the main ship was made official, where he was trying to convince Miyamura that Hori liked him. He got downright vicious. Sure, he regretted it after, but still. And there's the way he treats Shindou almost all the time, which is kinda where Hori's M kink comes from, because she got curious what it's like to be on the receiving end of that kind of treatment as someone close to Miyamura, who normally doesn't show that kind of side. In a way, I think it was like she felt like she was missing out or somehow not actually close to him if he wasn't willing to get belligerent with her at least once.
I think people also forget he literally got so pissed first time he met Sengoku he headbutted the guy. Yes, most of the time Miyamura is a sweet guy, but definitely from early on they showed he can get mad and violent himself, especially when Hori's the topic. Such strong emotions involved in protecting her. And she's expressed the same in kind.
Anyway, something else that's kinda clicked for me, thinking about it... these two have only officially started dating this last year of high school, right? Specifically the confession and such happened around summer (early summer? June, I guess?), considering chapter titles. And we're only starting to get into fall here. There's a lot of shit happening their senior year, like holy crap. And, in the grand scheme of things, since it hasn't really been that long, it's not surprising to have jealous moments while they might have doubts about each other's committment to the relationship with it being so new, still. It feels like it's been awhile for us audience, but it really hasn't been nearly as long for them as characters, so keep that in mind.
It's kind of funny, since the early volumes (mostly the first) felt like they were skipping huge chunks of time to imply they were getting close while Miyamura was coming over to play with Souta on a nearly daily basis, but the moment the relationship started, it kinda felt like time started to condense and lots of shit started happening in a very short amount of time, per chapter.
Like, think about it, The start of the story kinda felt like it took place early in their second year of high school, right? Then we rapidly skipped through that second year (the whole first volume feels like it goes through the whole season range, just about, from spring, to summer, to fall, to winter) up through the middle of volume 2, where they transition to being third years, and they've been third years for several volumes now but it's only just barely fall, I want to say.