Ok, just think about this line for a minute. Law probably has a lot of respect for Whitebeard for this. While he was with Doffy the whole “family is everything” mumbo jumbo was always being talked about by the Donquixote Family, it was a rule they lived by, or so they said. Then Cora-san and Law betrayed the family and Doffy didn’t even bat an eye when he killed Cora-san. He didn’t care. But then Law hears all these stories about the Whitebeard Pirates, how Whitebeard is their father, that they consider themselves a family. So Law listens to the stories and gathers information trying to prove them wrong, that it’s the Donquixote Family all over again. But he can’t find anything. Whitebeard is a legend for the exact reason Law says, he doesn’t sell out his companions. So Law, having experienced the facade that was the Donquixote Family feels a mass amount of respect for Whitebeard for keeping his word and having a real family as a crew.