"Why?" Castiel looked almost confused for a moment when she asked this, because it wasn't a response he thought would come from that. "Why do I care about you?" He clarified, for further reference so he knew what she was questioning him about. It wasn't an emotion or connection he could explain, because he didn't have connections that actually meant a great deal like the one he had to Meg, with too many people. The other angels in his garrison, his brothers and sisters, that was different because he'd known them for thousands of years. And then, for some, he only saw every now and then, rather than on a constant basis. So he only had his connections to the Winchesters to really draw off of and he knew what he felt for Meg was much different, in comparison to his connection to the brothers.
"You looked after me when no one else would. When I was broken and without hope, it seemed." He didn't mean he cared for her because of what she did for him, but rather he was giving an example of why shouldn't he? "You're so full of that thorny pain," he noted, bringing up again that he didn't take her prickly outer shell at full value and saw something else underneath. "I don't quite understand how human emotions work just yet, only being newly human but even when I was still in possession of my grace, I knew you were no longer the enemy." That the draw he had toward her was not because he had that instinct to smite her, to rid the world of filth, but rather because there was a connection between them that was difficult to explain.