The scar being gone is sad. I was already going with the theory that Castiel healed it when he healed Dean in 5x22. Yet it’s only adding to my Dean/Castiel feels—that when Castiel healed Dean, he wanted to heal all of him, and took away even the mark that Castiel left on him the first time around. Given the reveals of season 6, it seems like Castiel mistakenly thought that removing himself from Dean—including a reminder like the scar—was a kindness. He was trying to spare Dean any further pain, saw himself as part of the troubles that fell on the Winchesters. He wanted to give Dean peace; he said he didn’t want to ask Dean for help, ask him for more, after Dean had already given so much.