Then there was no flirting. It is a trait that makes him different from most. Where he learned it though, maybe it was time or from the Winchesters example. Faith is something he would likely more proscribe it to, but he could use to learn what it is called to humans. He has a long way to go still. However, he tell that she is pleased, which in turn allows him a sense of fulfilment before he's even delivered on the promise. "Tomorrow morning," he declares. She doesn't know it but she gave him something to do in the wee hours when he's inevitably awake.

It's not likely that he will ever lose his clueless demeanour, specially when he's lived much longer than the host he inhabits, but the years he's been on Earth and knowledge he's gained will continue to enlighten him. For instance, Lucy has gotten through to him with the reasons why he shouldn't eat toothpaste anymore. That's a feat even Sam couldn't do.

Apologies there. Honestly, Castiel has no idea what she is saying and he's learned enough over time to assume such language means some space is needed. Perhaps he needs a new teacher. When she looks pleased, he finds himself even more puzzled. "Yes." Did he not say that clearly enough? He goes over the exact words he said in his head, and what she means to him to find an answer for her. "I'm not sure there is a word that describes what you are," he decides. Because friend is close and lover ("girlfriend") isn't quite where he understands it is.