It's not that Fallon is being protective of Lady.
He had a lot of experiences of being approached by people with hidden intentions. Because of that, he's wary of those who'd try to do the same to her. It's also not his fault that others are easily intimidated by him. He just had enough of fake smiles at this point.
"I'm only looking out for her, okay? She gets so upset whenever she loses a potential friend. That Snobby Rich Boy better be worth it. He's not going to get away with it if it turns out that he's only leading her on," Fallon muttered, earning an amused look from Belle.
He just huffed once more, certain that she misunderstood again. She didn't know about what happened to Lady back then. He could still clearly remember how much she moped around after that particular incident. It took a while before he's able to fully cheer her up.