After a botched witch hunt, Cas ends up an infant; it's strange to see their once all-powerful, indestructible friend so small, so helpless.
It's even stranger to see Dean look at Cas like that--like he's cradling the whole world in his arms.
Sam's trying to track down the witch, or find a cure, to get Cas back. But it's kind of difficult with all these intruding thoughts that never go away; the kind that make him step back and question everything he thought he knew of Dean and Cas's relationship.