I completely understand why someone would not like this. A lot of parts of the story and the way some scenes are forced is pretty mediocre. After a few chapters, it got kinda lame and the new characters that appeared didn't seem to contribute anything to the whole. But this manga is also really fascinating. Or at least the reason why some people get really attached to this kind of history and relationship is fascinating to me.

Nagatoro is a school girl who likes to bully her Senpai. Simple as that, that's the whole thing. She plots different things to make him suffer and seeing him shivering makes Nagatoro thrill, using everything at her hands to do so. Her strongest desire is to be close and to have him caught right between her hands, to make him feel like his life is controled by her. She pressures him to do as she pleases and that brings her joy. He's her little toy and she loves to play game after game.

Senpai, only called that -we don't need to hear anything more, really-, cries, gets insulted and is about to breakdown in almost every scene. To call this an s/m relationship is kinda fishy as he is only tolerating and acepting this because it is easier for him (he even explains it and his general relation with bullying).
He grows to like Nagatoro and their relationship, yes, but what Senpai likes is not being teased or attacked in any way, he doesn't enjoy being put to shame, being uncomfortable or being ridiculed. The pain and suffering are not arousing to him. What he likes is all the attention she gives him. It's very questionable how healthy this relationship is, some people probably don't care about it, they just want to read something they enjoy, but there's a bit more to it.

Suddenly, a younger girl spends a great portion of her time with you and has her eyes fixed on you. She talks to you and touches you. You think about her. She's there, she's real. And you love that, 'cause you never had anything similar to that. It's incredible. You even wished for something like it. It doesn't matter under which condition it is happening. You feel hers.

Nagatoro gives herself to Senpai in a way more honest and, you could say, pure that the one perceived in traditional love. Bullying seems to be more direct and they share pleasure through that, something that seems very resonant with people, specially young men. She wouldn't leave Senpai and wouldn't let him escape either. There's certainty and comfort in it different from the one in common romance. Then, Senpai holds into it and gets himself absorbed. He feels hers. He feels someone cares for him due to all of this. And that's what matters to him.

Through the chapters, something similar to a love relationship normally depicted in manga seems to emerge, but you can't ignore what was the base for that development. After all, it is what I found interesting about this, the illusion behind their relationship.

PS: I don't really care about the art, sorry.