This is Dean placing a quiet kiss on Cas' forehead to help him through the rough bits of being human. In the quiet it's easier to remember what you were..but you can't wrap a human mind around being a crackling front of intent, of power waiting to manifest at the word of your father, in a screaming blinding form that would be toxic to this small, temporary man in front of him who grabs bits of life so fiercely, and shuts himself off from many others. Maybe he can give him something too, when he learns to be a person. Maybe he can give back to Dean.