We are family.
He said that to Cas. It sounded odd even to his own ears. But Dean doesn't have any other word to describe what Cas means to him.
Dean knows it's not right to call Cas 'family' in the same way as Sam and Bobby. Sam is his flesh and blood, and Bobby has always been like an uncle to the Winchesters. But Cas? Dean has known him for only about five years, during which the guy was often missing. This guy isn't even human. He's a freaking angel - some thousands years old - who has served Heaven his whole life.
And yet, Cas managed to become his friend, and then before he knew it, something more than friend.
Family is what Dean has cherished the most in his life. In Dean's book, family is the people he will never give up on, no matter what happens. Family is the ones he can't stop caring about even if they hurt him badly. And somehow, as odd as it sounds, Cas falls into that category.
And now he's missing - again. At least this time, Dean knows he's out there somewhere. So he silently prays that the angel will find a way to come home.
End