If his planning to get married had caught her off guard, his answer just now pretty much knocked her off her feet. (Not quite literally, of course, but it may as well have with the way her heart jumped.)

"Are you serious...? You're serious aren't you!?" Oh, god, whatdowhatdowhatdo-- "You can't joke around with a maiden like that! I'll kill you if you're just joking around!"

Because the moment he said it, she found herself liking that idea, though she wasn't quite ready to admit it out loud yet.