He isn't thinking about how closely he's resting against Greg's body - right now it's a much needed bit of comfort that he isn't going to examine too hard. Maybe Greg is right. But there's an important difference in context that he doesn't have any answers for.

"But is...is he really someone else? Why do...why do I remember that, why do I have the same things? If I was actually like that before this isn't okay."

If it was just an image of a life that could have gone better it'd be one thing, but so many of the signs point to it being something that really happened, to him personally, that was...changed somehow. If that's the case, he doesn't know how he can stand what's been done to him. He doesn't even understand why it was done in the first place.