Erm are we really reading the same manga. Shima is always wary what other thinks of him that's why he hardly reject anyone's request or offer. He always put himself in good mood so he doesn't bother other people. And by understanding that BEING HIMSELF doesn't mean other will hate him, is a character growth.
Oh yeah my mistake. I meant he usually didn't show that much interest. Back then when things got awkward between him and Mitsumi he didn't care much if they would end up growing apart. He believed that people come and go. He is a man pleaser since he doesn't like hurting people. But still, he has that somewhat 'ah fuck it' personality.
I think that he repressed his emotions for so long because of his past, that he didn't even knew how to start contemplating about them. Shima made himself into a mold to suit other people needs since he feels needed that way, because if he started to think about it more, he would only spiral into self-hatred even more. So when he had to confront what he actually feels, he started to overthink or make mistakes. More than being shaken by love, Shima started to notice (thanks to Mitsumi) that even when he is being lame or when he doesn't mold his personality for others, people won't hate him.
(Then maybe he will realise his love for Mitsumi cmon im waiting for that sweet romance<3)
Actually i'm not even sure if what I said is correct but I would like to think that it's like that. (●'◡'●)ノ
OWEMJI HE STARTED BEING WARY FO EHAT OTHER PEOPLE THINKS OF HIM!! HE USUALLY DOESN'T CARE ABOUT THINGS LIKE THAT!!! NOW HIS RESOLVES IS BEING SHAKEN BY THE ONE AND ONLY MITSUMI!!!! LEZGO MITSUMI!!!!