Castiel just desperately wanted to lose himself in the feeling of the kiss, something he'd missed so much over the last few months. For a long, painful moment he could almost believe that the betrayal hadn't happened, none of it. That they were still together as he kissed her.
But it had happened and they weren't together. And still kissing her was just going to end up badly for both of them.
He pulled away, and if he'd been human he'd have been short of breath. For several seconds he didn't say anything as he waited for his brain to catch up.