Responses
I agree.. to me it just seems like he doesn't wanna live that kinda lifestyle anymore and doesn't know how to go about changing while Misuzu seems to be adjusting to his new life smoothly and has someone who cares for him and is there for him other than Sanada. While Sanada is alone
SPOILER FROM ANOTHER MANHWA CALLED "LOVE REVOLUTION"
DON'T READ IT IF YOU DON'T WANT TO GET SPOILED. DON'T READ IT IF YOU DON'T READ THAT MANHWA
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Idk if it's a "romantic" jealousy type of feeling.... But so far what i got from Sanada's character is similiar to Choi Hanbyul from a manhwa called "Love revolution", i think the jealousy feeling is similiar. In that manhwa Choi Hanbyul is not really a bright child growing up and got bullied a lot before and the FL (wang jarim) is the one who helped him through all that basically like a "superhero" for him maybe, Jarim met Hanbyul again when they're in highschool (different highschool btw). Ever since Hanbyul got a glow up, people always swarm around him but the only one that knew his miserable side is Jarim.
Well of course in ML's eyes (Juyeong), Hanbyul is trying to snatched Jarim away from him but it's not like that, well maybe Hanbyul is the reason why Jarim and Juyeong got into a lil bit of fight but deep down Hanbyul is jealous of Jarim's life rather than jealous of Juyeong. He just want to asked her "how could you enjoying life like this after you practically left everything behind?". At some point Hanbyul turned into "someone else" just to fit in, similiar to Sanada maybe in this story.
Hanbyul feel left behind meanwhile Jarim just continue with her life (i think they also lose contact after kindergarten or elementary school). At some point he also wondering why Jarim wanna be with someone like Juyeong and lookin down on him, but in the end he understand why she choose him. He's jealous of Jarim because he got someone like Juyeong (and their friends of group) that actually UNDERSTAND her, where she can become herself in front him without gettin criticized, without become "somebody else". In Hanbyul's bubble there's only Jarim, that's why he feel "safe" in his own bubble. Jarim promise to be with him and yet Jarim "left" him (that's from Hanbyul's POV btw) and no one can enter his bubble again after that. So when he finally met Jarim again, he expected Jarim to be in the same circumstances like him, but what he got is completely different.
If i remember correctly (since i read it a long time ago) what Hanbyul ever want is just a simple sorry and acknowledge from Jarim and a place where he can be himself without gettin afraid of other people's opinion (that's why despite the bickering between Juyeong and him— more like he ragebait Juyeong and Juyeong fell for it, he actually feel more safe with someone like Juyeong who got no filter)
I think in this case, Sanada is jealous to see that Misuzu can actually keep going with his life and ofc got a boyfriend that actually UNDERSTAND HIM... Idk about romantic feeling tho, it could be that, or not.
So it's more like, in Sanada's mind Misuzu is also similar to him, someone in the same circumstances. So what he got in mind is "oh... We can be MISERABLE together", in his bubble, there's only Misuzu. He met Misuzu again but, everything's is different now, what he got was Misuzu is actually living his life with someone that he actually loves, he didn't need to pretend. Sanada feels like... it's only him that stuck and walking in place while Misuzu is blooming to the better version of himself.
I don't blame them tbh. They're teenagers after all :(