[She understands why he's upset, knows that even if he doesn't quite seem apt to admit it that he cares a lot about Spock and wishes things were different between them. There's nothing going on between any of the three of them, so there should never be any reason for raised voices, but it makes sense that he'd be upset given that he obviously thought she was hiding this from him.

And maybe in a sense, she was. She words her explanation very carefully.] There is a reason, Saval, but it's not what you think. You know how close our families are. Our parents wish that we were involved, but we're not and neither of us has any romantic interest in the other.