It shouldn't be possible, for two fundamental reasons: He was an angel, and as one, he wasn't even supposed to fall in love.
The second, and even more important... She was a demon.
A demon! How could he experience anything but disgust toward her? At most, he could form a tense alliance to achieve a common goal, but that was it.
Instead, he had fallen for her completely, while she took care of him. She was, in ways even he couldn't comprehend, as much of an angel as she was a demon.
She kept trying to pretend she didn't share any of those feelings, she rolled her eyes and looked irked when he asked how she was doing, if she was hurt.
She hissed at him when he praised her.
And yet, bit by bit, she had let him in as well. They had laughed together. And he wanted so badly to kiss her again, that was the truth.
She was beautiful, and not just the body she occupied. There was something dark and gorgeous about her, something most demons just didn't have.
Castiel was in love with Meg, and though he struggled with this fact, he refused to pretend it wasn't true.
It was wrong and messed up, but at the same time, it was the most earnest emotion he had experienced since arriving on earth.