Nagatoro has always been like that and he knew that getting into a relationship(they have been at least acquainted since he was in second grade and she was in first if i recall) whether we think its good or bad he knew her personality to the point where it even surprised him for her to back...
He doesn't not like it though cause he knows she is like this and from what we saw that seemed to be the first time he got defensive about it, which surprised her.