Honestly, I'll talk about this to the moon and back, but for me it's always their complete understanding and acceptance of one another and how they don't need to say anything at all for the other to understand what they're thinking and act accordingly. They understand each other's emotions and thought process so well it just makes me crumble to pieces dude. There's nothing more lonely than being a genius because you feel so isolated and different from those around you, nothing stimulates you, and then all of a sudden there's someone who GETS you and doesn't get mad at you for how you act. The sheer excitement of that can even be seen in how Dazai reacts to first meeting Ranpo and how the two interact so carefreely and energetically. They truly can be themselves with each other more than anyone else and I love them SO MUCH