"I wouldn't mind if you'd ever bother to wash it out," Stefan grumbled, entirely on the principle of having something to grumble about. "Plus, I already bought you a new one, so why're you still using mine?"
If he'd actually cared about it at all, this could have become a very serious issue.
As it was though, he was simply indulging in the pleasant sport of winding his packmate up slightly.