As soon as Cas heard the full story of everything that had gone down, Dean fled to his bedroom (something he wasn't alone in - his mother and Sam chose to do the exact same thing. They were all grieving tonight, the loss fresh in their minds). And as much as he didn't want to cry any more tonight, a few tears slipped down his cheeks anyway. "Fuck," he sobbed. He missed his dad so much and seeing his parents together had been mind-blowing. Sitting down for dinner as a family of four was something he had only dreamed about for thirty-six years and now it was a reality. Winchester Surprise had tasted damn good too, even though he was definitely going to pay for it later.
Dean was glad to have gotten closure, though, and hearing his dad tell them how proud he was of his boys and that he loved them killed him. Maybe grieving alone wasn't his greatest idea but he didn't have the energy to even move at the moment since seeing his father had re-awakened everything he spent years burying.
He'd remember this day for the rest of his life and cherish the memories forever, however.