↳ This- it won't last. You are living in a dream world.
I was talking to Miri ( laoih) about this scene, and she pointed out that right after he hears that, he touches his left palm, the one with the scar that Dean sewed up, that always brought him back to reality. Sam hears that, and he knows, he knows that in a way, he lives in dream world, and that it won’t last, that he needs to get back to reality. To a painful world in which he has lost everyone he ever loved.