When Anna takes a step backwards, Castiel is almost surprised and he wonders what exactly he was expecting. In theory, this was a perfectly normal, albeit honest conversation. There is nothing to differentiate it from anything else they have previously discussing. If anything, they at least know where they stand with each other and he supposes that is preferable to not knowing. "There is no need for an apology," he replies quietly, choosing to continue walking. "I am honoured."
That is also true, namely because Anna's approval was something he'd sought back in Heaven. She had been his leader and a symbol of some form of hope, though now he wonders what form of hope. She was the one he'd admired, however. He'd wished to emulate her as much as possible and become an angel of her stature. In some ways, he feels he's achieved that, albeit in perhaps very different ways. In the beginning, he had not set out with the intent to Fall, though he does not regret his past decisions.
"We...have become central systems of support for one another. It makes sense."
He is not certain whether he is feeling disappointment or not. He doesn't feel it's the time to assess such a thing.