Kylo isn't certain he can promise not to be stubborn about anything. But beyond that, the question throws him a bit, even knowing how she'd reacted a moment ago when he said he hadn't tethered. And how it seems like she'd come here purely because she'd picked up on his distress through the Force.

"You're worried about me." It's an observation, and his tone carries a note of surprise about that revelation. Not so much that she does care about his wellbeing, but that she's unusually willing to make it known right now.