Cas ought to talk to Ruby about it. She can help him research it and not worry anyone. Dean is glad. He's pretty sure that Lucy is more careful than that, but it's definitely something to discuss.

Good, he's not disappointing Cas then. Conversations with Dean are complicated and labyrinth-like. Dean contemplates that a moment, frowning as he thinks about it. "Think it has something to do with this place?" He could mean Cas's rocks or the humanity. Take your pick.

Dean likes Lucy and as long as Lucy isn't breaking Castiel's heart (which he thinks could be likely given what he knows about Lucy)then Dean is happy for Cas. Lucy is already a bit like one of the family. All the better if she's one of the family and Castiel's girlfriend.