[ That is a great reaction right there. He was worried for a moment, that she'd protest or make this into something it isn't. That day is so far off and they've barely scratched the surface of this place. For all he knows, one or both of them will be gone long before he has to make that decision.
Maybe that's why it's such an easy promise to make. Hollow. If you can't leave someone, it's easy to say you'll stay. ]