He doesn't understand why you care about your "property" so much. She's just a mudblood to him. You can always get another. To care that much is unnatural. So now he's decided that Hermione must've done some kind of sneaky trickery to make you care.

I don't know what to do. Maybe everyone else has ideas, but all I've thought is that Hermione might need to tell me a real secret. Not that she does magic, of course. But something real and believable. I hate that it has to be that way, but I don't know what else will work.