Yes or no?

[Ruifeng]

A: That's just about enough. I am leaving.

B: Oh, what's the matter? This doesn't interest you?

A: Oh, no. It's fascinating...

I thought A is trying to say, "Oh, yes, it does." Right? Then why is "no" used instead of "yes?"

Thanks.

[Mister Micawber]

'Oh no, your thought is not correct'
_________________
Native English teacher at Mister Micawber's