I think Data came to 'that' conclusion because it actually made kind of sense to most of the crew considering none of them really knew how Q was around Picard when Picard and Q were alone. For exemple, all that Data had seen so far from their interactions were Qs teasing, Qs introduction as Robin Hood and everything else he had only heard from Picard. And I doubt Picard ever told any of his crew that Q appeared in his bed twice. And then there is the clear difference in evolution, which I think is also something Data is aware of and factoring in. After all, Q is a being far above anything organic and of a life form for which the 'laws of physics' are only suggestions at best. It would also not be amiss to assume that the Q exist on more than just three dimensions and he is basically immortal and already so old that a humans life span should mean nothing to him. So, to Data and some of the crew, any fondness Q has for Picard would evolutionary be more equivalent to a human being fond of a small fish or, even worse, an amoeba.