Mary had complications after having Dean. The doctors didn't notice it right away, and Mary, not wanting to stay in the hospital any longer, took her baby boy home. Castiel knew of the health concerns, and so he traveled back in time to help Mary. He heals her while she is laying in bed, and goes to leave, but before he gets the chance, Mary starts incoherently asking about Dean. Cas tells her he will check on him before putting her in a deep and peaceful sleep. He walks into the nursery and looks down at the tiny bundle fast asleep in his crib. Without thinking to do so, Cas reaches down and picks up the infant, cradling him in the nook of his left arm. Sensing the movement, Dean wakes up, and stares up at Castiel with large bright green eyes.
"Hello, Dean," Cas says, and the baby gives him the biggest smile accompanied with a loud laugh. He starts swinging his arms at Cas's face, reaching up at him, and Cas stills his small arms with his right hand. Dean grabs his thumb with both of his tiny hands and immediately tries to put it in his mouth. "I don't think that's a good idea, Dean…" Cas starts, but as per usual, he lets him do what he wants. He stares at Dean, who is now sucking on his thumb and cooing, and smiles at him. Castiel amuses himself by picturing how present Dean would react if he could see this moment.
Dean looks back up at Cas, and Cas gazes back into his soul; it is bright and powerful, whole and undamaged, untouched by sin; a destiny with a magnitude Cas knows he will never truly appreciate. Cas's smile fades as he remembers everything that lies in wait for the infant in his arms. The fear, the hate, the pain; it will all be starting in a matter of years. Dean was starting to fall back asleep, still holding tight to Cas's thumb.
He stayed like that for a while, just watching him. He had noticed that Dean was always most at peace when he slept, which was why Cas always hated waking him, preferring to just stand by and watch him until he came to on his own. As carefully as he could, so as not to wake him again, he laid Dean back down in his crib and stood over him. "Don't worry, Dean. I'll watch over you." With a flap of his wings, he vanished.