"People will forget what you said, people will forget what you did, but people will never forget how you made them feel."
There is more to literature and story-telling than an intriguing, "flawless" plot. The best stories are not the best simply because they are the most interesting. They linger in our minds long after we have read them because of the depth to which they drew us into the experience. Whether it be through excitement, disgust, affection, or sorrow, the greatest works are able to latch onto our emotions and connect us to something that becomes much more than just a piece of fiction.
Japanese writers have long been infatuated with the tropes of death and character growth. Mangas that develop around these concepts are far from cutting-edge. Sundome isn't considered great because it brought new ideas to the table. It is a masterpiece because it moves and drags us into these themes to a level that very few manga have ever achieved.
How Okada Kazuto is able to connect us to his characters and his story to such a high degree can't be explained with words. It is an "act of God." Perhaps it was the way a certain panel was drawn, or the way a single thought bubble presented itself. Sundome begins as what seems to be nothing more than comedic erotica. By the final chapter, however, we have grown attached to the manga to a measure that may catch us off guard.
Kurumi is never protrayed as "weak", nor does she ever anything to forcibly invoke pity. But we learn to love her. Its hard for us to come to terms with her death, and its even more difficult for us to see her leave Hideo behind. Her passing, like the sundome, is tragically beautiful; her short-lived existence only serves to impart gifts and beautiful memories to Hideo. Our thoughts echoes his at the end: Why him? All we can do is stare at this unlucky girl and admire her actions, born from an unconditional love that is as unfathomable as it is powerful.
In conclusion, Sundome is arguably the greatest tragedy manga ever written. Some (unfortunate) individuals may not be able to fully connect with the story and experience it. But for those who do, it is something truly special.