Sam already knew. Of course he knew. But that doesn't change the fact that he feels like all the air got sucked out of the room. His head is dizzy, his chest is tight, and his legs are weak. If he wasn't already sitting down, he'd surely have fallen down.
He wants to believe Dean. He hates this doubt within him. HATES IT! But, how else to explain the shifter taking Amara's form?
Dean can see the wheels turning in Sammy's head. He can feel the dread coming off Sammy in waves and it kills him. Dean wants nothing more to assure him that he has nothing to worry about, but that'd be a lie. They both know it.
"Sammy. I'm connected to her. I am. I fucking wish I wasn't, but... I am. It's real and it's strong, and it very well may be my undoing. For real this time." Dean can't help but look off to give a sad little chuckle at that. Then he makes sure to look Sammy straight in the eye for this next part. If Sammy only believes one thing Dean says, please God, let it be this.
"But it's not love. It could NEVER be love."
Sam has to fight back the tears in his eyes and the lump in his throat as he hears Dean's words. After a moment, with what feels like his last breath, he's finally able to form a reply. It's no louder than a whisper.
"Really. That's just for you Sammy. Only you."
And just like that, Sam can breathe again.
P.S. "Well, only you and Cas."