Fu Shizhe's best/childhood friend, Jiang Yun, committed su*cide while they were both studying for their doctorates. Jiang Yun was like a brother to Fu Shizhe, so his death impacted him hard, especially since there were signs of Jiang Yun's unstable mental state. At the time Fu Shizhe could sense something wrong, but did not understand the extent of Jiang Yun's inner turmoil as he had kept it from Fu Shizhe. Therefore when Jiang Yun dies, Fu Shizhe reflects on things he overlooked. The MC and ML met 2 years after his best friend's death, so understandably ML is still very affected by what had happened.

I think the worst part is that Jiang Yun's death occurred right after his talk with Fu Shizhe. Not only that, but the same building as well (Jiang Yun had jumped). The Jiang family had asked Fu Shizhe to make sure Jiang Yun takes his medication (he was diagnosed with depression and anxiety) after Jiang Yun had c*t his wr*sts. Fu Shizhe did so, also out of his own concern, but Jiang Yun was spitting out the medicine secretly (Jiang family blamed Fu Shizhe when news broke out, which left ML feeling as if he is the one to cause Jiang Yun's death). Fu Shizhe had found out about Jiang Yun's diary entries before his death, where Jiang writes about his feeling of incompetence, stress of school, being taking advantage of/blackmail, wanting to drop out but parents disapproving, envy towards Fu Shizhe's intelligence etc. Overall a lot of bottled up emotions on Fu Shizhe's side with him feeling heavy guilt/remorse.

For the most part, Jiang Yun considered Fu Shizhe as his younger brother. It is only when Jiang Yun's mental health took a dip for the worst that he starts to become jealous of Fu Shizhe and compare to him. Jiang Yun thought that gradually getting off the medicine would make him normal again, but we know that is not the case.

he's so asocial and withdrawn and I think it's related to his past?

one example is how she figured the ML refused her advances because of his words "I don't want to fall in love (for now), " and she was, understandably, very sad. but her sadness lasts for such a long time it will make you think that she has a hidden illness and will d!e soon if her confession is not accepted instead. at least that's what I feel from her emotion and thinking process. she's also kind of wishy washy and really like making herself feels pitiful with the "he doesn't like me, but I love him so much, he's the light of my teenage life..." mindset.