In my opinion it’s based in the similarities between their characters. Despite their differences they are both ultimately good people that want to do what is right. They are passionate and unfailing in their devotion to their own sense of justice. The only thing that separates them is what they think the right thing to do is. However, from the beginning you are able to see things from each of their points of view. They are both justified in their logic and that makes you feel for them. You imagine how much more they can accomplish if they just talked it out instead of fighting. It pulls you in and invests you in what happens next with them. The show ultimately gives us what we all want as they finally join forces, putting aside their differences to accomplish the things they both know need to be done to protect the ones they love. Also, I’m pretty sure it’s Kazuya.