bro i've been reading this shit for so long that i forgot how they fell in love..
In summary he gets raped. Then kidnapped by a "more scary" rapist in another country. He is brought back to "safety" giving him a false sense of freedom and creating dependence to his original rapist. Then a jealous whore appears and makes his dependent stockholm feelings spike making him more attached then he was before. His rapist now being more protective of him gives validation to said feelings.
As for Asami it went from curiosity to a challenge (including trying to break him) to a thrill (not quite being able to) then loosing that thrill to someone else then getting it back and now having the urge to preserve it as he first found it so he never loses the feelings he gets from it.
chapter 85 was so beautiful ╥﹏╥
We'll either see them get married and grow old together or one them will die and this is a story he's been telling his grown up kid. In that case this might be je-oh who lives.
edit:I meant the kid they adopted together. I'm just guessing,hope the second one doesn't happen. Hope the author ends things with a wedding and a kid adopted. Happy ending ╮( ̄▽ ̄)╭