Castiel's not certain why she begins laughing, but he knows it is not a laugh of ridicule. If anything, he is pleased to see her genuinely smile and honestly, it is a pleasant feeling to know he is the one who elicited such a reaction from her. He may have made some form of error, but that does not seem to matter. "Like Anna," he replies, logically.
There is a very faint smile spreading across his features, unbeknownst to him seeing as he's never considered such a thing before. With emotion as foreign to him as it is, he barely registers that he too is amused, albeit in his own way. "Though...perhaps warmer. Was that the correct action to take?"
When his eyes meet hers, it's difficult to tear them away. Something has changed between them - is still changing - and has been ever since they began this new life together. He cannot explain what it is, but he feels he likes it. How to explain that however, is another question.