"You're telling me my entire life is made up?" He looks offended, because how else is he supposed to take that. What, none of his experiences are real just because Jack says so? Sometimes it drives him crazy how tactless he can be at times, like he's so used to being right he doesn't have to care about what other people think of him.
And frankly he's not sure he believes it anyway. So far everything that's happened here has seemed pretty out of character for him. If it's Jack's word against his own memories he's going to trust himself, thanks.