teppogaijin , no, they don't make each other happy, they make each other miserable and use each other, expecially kitahara. All she did this whole time was use him to get her revenge. They have a mutual goal and now a mutual pain source.
I don't think it was foolish of her to think another boy would be happy for her finding love, as she saw kurosawa as a friend and thought he saw her as one too. You can't expect someone to guess your feelings... plus, as he said a few chapters ago, she smiled and talked with others the same she did with him. He was a closer friend to her, sure, but still just a friend. And honestly, it's both sad and extremely relatable that he can't keep being friends with neither takagawa nor nagaoka...