"It was the easy choice. They both were hurt by it in the end, but walked away from one another." Castiel looked at it in the literal manner, because they gave each other up. It wasn't entirely easy, but they didn't have to fight to keep anything. "They gave up." They might have ended up in a concentration camp or worse, dead, but it didn't mean they wouldn't be happy before that. And from his understanding of human relationships, that was a feeling that some people never truly experienced with another, not at least when it came to love. "He should have included her in that decision." That, he learned from his failures and the repercussions from his failures. The way he had to atone for them.
"They only loved each other in their memories. And that is sad." From what he saw, they would have loved each other, yes, but it would have been in the past tense. They loved each other, as a memory and had to move on past that, possibly not feeling that way about anyone else beyond that.
If only he were aware that Meg was using their conversation about the film as a framework to truly talk about their own situation. The feelings that they both had yet to name, because naming things made them real. If they named them, then they would have to face them and they were both unsure of how to truly approach that, despite that spark being there. The very spark that made him eye her curiously, his gaze trailing along her features, to that barest hint of space between them and how they were barely touching, yet he still clearly felt her there next to him.