FMC is an annoying, cowardly tsundere who is reaping what she has sown. She kept rejecting his sincere advances and confession, and now is upset and confused that he has given up on her? As much as I can't stand his simping, I do still have sympathy for the MC.
He appears to have grown-up in a fatherless home, where his mom and sister constantly belittled him and ground away at his self-confidence. Even if that wasn't their express intention, it is still what happened, and is borderline abusive. So he is no Chad to begin with, but the MC carries on and finally gets the attention of the cute girl in his class that he is crushing on. He then spends almost two years trying to get her to reciprocate his feelings, his blind infatuation of her keeping his sense of inadequacy and negativity covered over. He has effectively created a youthful fantasy for himself, where eventually he'll get the girl and they'll live happily ever after.
Until one day, something snaps him out of it, and in trying to come to grips with reality he ends up sliding to the other side of the spectrum: pessimistic self-loathing and distrust of others. While not a sociopath or psycho, it's pretty clear to me that he is broken to some degree. He has resigned himself to being just a mediocre loner that no one really cares about all that much, admiring the light of others from afar. Fortunately, people around him have taken notice of this change (tomboy friend, his sister, and even Natsukawa) and/or are concerned about him (ponytail-senpai and loli-senpai plus the student council reverse harem). Still, he lacks the inner strength to shrug off all the negativity that he has let affect him, so it is going to take a lot of positive reinforcement (and some effort on his part) for him to start seeing the silver lining again.
I just hope the author/mangaka can portray that in a satisfying way w/o the story feeling like it is going nowhere for chapters on end.