Castiel still isn't sure what to say or how to say it. For all Dean is hard talking about these kinds of things, Cas is even worse for lack of practical experience.

"I didn't know what to say, I didn't know how to bring it up and even if I had I would have expected you not to want to talk about it." It is Dean after all. "You have always dealt better with actions than words, so I thought I could show you instead."