I was thinking today about why Meg liked Cas. I feel like she liked him because, to her, he sort of represented her redemption. As she said, she became "kinda good." She fought with him, took care of him when he was in the hospital, and ultimately died for him. A demon, having done countless horrible things, somehow found herself with a friend. Sure, he wasn't completely sane, but he cared about her. I imagine Meg had probably never had anyone care for her, let alone an angel, of all things.