Cas doesn't even deign to acknowledge the featherhead comment. "There is no one else," he answers flatly.

"But I see now I was wrong," he goes on, flatly because it's the only way to keep from breaking in front of Dean. "I should have known this would never work. That you and I could never be... this." His voice shakes near the end, and he falls silent.

He doesn't know now why he ever thought he and Dean could have something here that they don't have in the real world. Destiny, true destiny, can't be overcome.