I can try! It seemed Zaizen fell for him over time as Kiriya was always there for him when he needed him the most. Plus Kiriya was good to his sister who is his number 1. Plus, Kiriya is and always was a hot guy. Kiriya fell in n love with him when he was young simply because Zaizen is beautiful. It took him a while to understand his feelings for Zaizen as he was a Yakuza. They can't be gay. They would be killed by their group or cast out at the very least. Now he just can't control his feelings anymore so he is going after Zaizen. Zaizen though is trying hard not to give in due to the legalities and views of gays and yakuza in Japan. Hope it helped!
Sorry but i have many question about couple Zen and Kiriya. Sometime this relationship make me confused. I don't understand about Kiriya's love to Zen. It's very difficult. Someone please explain to me because i'm not good english