Harry didn't want to agree with Snape, even though he knew he was right. Harry and Ron haven't really been the same since fourth year, when Ron stopped talking to him after the whole Goblet of Fire incident. Harry knew that he had been pulling away from the Weasleys for a while, and then even more after he had come out to them and they realized he wouldn't be marrying Ginny after all. A small part of him had known that they had clung to him for his name and fame, but Ron had been the first friend he had ever made and he hadn't wanted to lose that. So he had convinced himself that it was all in his head.