"Without knowing what this is, it's not a promise I can make," Klaus points out, because he has to. He doesn't want to hurt her, although he suspects his words will, but lying to her would hurt her more in the long run.

And he can't promise her, because there are some things that he couldn't forget, even from her. They couldn't be this in 500 years if she's betrayed him by then, a true betrayal, something worse than she's done for now, after he actually comes to trust her.