It really is an extremely unique romcom writing in a shounen manga for characters to actually act realistic. It's not even so much about
Here instead of the usual denial and failure to understand things (the infamous "dense MC" trope which from once a fresh comical relief with time turned into almost a slur towards cookie-cutter harems) we have both protagonists accepting their feelings—although being understandably unsure about what to do with them or just how real they are—and side characters who which are actually able to not only notice things, but also to act like proper humans instead of yelling it out the first moment like some satirical cardboards.
Yeah, it's shounen all right. Well, "modern" shounen, which are quite open about semi-sex-relared themes like masturbation compared to the works from the beginning of the century or even more classical ones.
But this "shounen all right" is literally one of the best out there in turms of realism. Many seinen works don't even come close to the result of all subtalties and little moments that we get from this series.
It's basically on the level of mature jousei (think Nana), which is essentially proper romance literature level (although there's only a handful of proper jousei for dozens of grossly overly grotesque yaoi).
Before this work I was almost skeptical towards shounen romance (which are almost always just unrealistic stereotypical rom-coms), with even such pleasant and mellow works like Soredemo Ayumu wa Yosetekuru being just simplistic representations sustainable only in a bubble.
What can I say. Even shounen can sometimes pull it off like a pro so even adults are engrossed.
Sasuga, Sakurai-sensei.