@Pantropicaloux there isn't a reason for him to like her. This is not a romance story, there is no "like" between them. You to take in cosideration that this Fu guy is a psycopath, he doesn't like her, he is just obsessed with her.
I believe it was clear since the beginning that he just wants to torment her because of the incident that happenes in the past that got him caught and improsoned. His goal was to make her suffer. However He Yan kept on fighting back in ways he did not expect. "Playing" with her has become a source of satisfaction for him. And throughout these gambles they take with each other Fu has come to subconsciously think that he owns He Yan.
Why does he want her?
He made her divorce her husband, and forced her to be with him not becuase of like but because of obsession. He is a psycopath, people like him are narcissistic and has inflated ego. He believes that she is his possession and what's his can only be his until he doesn't want it anymore.
Yes, the reason is quite petty, and it's not meant to be romanticised. That's why I like this story knowing it will not end up in a Stockholm syndrome. There is no excuse for Fu's terrible behaviour.