To people asking: he's jealous. There are multiple hints, some very explicit, that Dani likes YCY. And she's liked him since before when they were younger. That time he told her he liked her because she's not interested in him, she already liked him. As for BYD... he's too pitiful. Dani does have a slight crush on him, as one would do to such a perfect guy, but she's not in love with him. Deep down she just thinks just that it'd be incomprehensible for someone like her to turn down such a perfect guy. She thinks he's the only guy she's allowed to have as he's the FL's brother and all the other boys belong to BYR or to other FLs. I'm not saying she doesn't like him at all or doesn't care for him, but her feelings aren't exactly love either, maybe because she likes YCY instead. And BYD noticed her feeling for YCY :(
I haven't finished the book yet, I'm on chapter 523 of the translated version on this website. But I already knew about the ending a while ago and while reading the book I got sadder and sadder, just from knowing it wouldn't be a completely happy ending. Finishing any beloved novel is always sweet-and-sour, but the ending of this novel is especially bittersweet. I haven't finished it it, but already know that I'll miss all characters and feel sorry for them as well.
Thank you for the chapter!~