It’s a known fact Yamada notices quiet girls laughing at his jokes. It’s the unpredictable nature that you assume quiet people just keep to themselves and are in their own bubble. But when a girl he doesn’t expect laughs at his jokes he gets awed even if he doesn’t know it. Unconsciously he starts thinking about the person, he’s kind of oblivious to his own feelings in the matter. And this is currently happening with you.