Keith almost says Isn't there a dragon out there somewhere? But he's grown a little, in his time with the Blade, so he manages to keep that to himself. He knows Shiro worries, and he wants to shrug it off -- even after all this time, he's not really used to anyone worrying. Shiro's been the only one who has, since he was really young anyway.

And Keith gets it, in a way. He worries a lot about Shiro, after Keberos especially.

"I'll be as careful as I can, Shiro. I promise." And he does mean that.

He also means what he says next, "So... worry less about me and more about taking it easy until you're patched up."