Ok I'm still reeling but after a deep dive into Destiel Twitter ad Tumblr I'm really getting it. I've watched the scene 70 times now and I understand Dean's reaction. He doesnt believe he deserves someone that truly loves him. He repressed every feeling towards Castiel and was in shock when Cas said I love you. His reaction is actually perfect for his character. It's just like him to not say anything, he finds every way to not show his emotions. This time it was just too late. I really wish he could have said it back. I'm really feeling the impact of the scene and hope we get more closure in the next episode.