The Master recognized the care - no love - Jack was putting in this. Although he was used to it from a completely different man. He couldn't truly give it back in kind, but oh, he would accept it. And give back what he did have, which was honest respect for Jack's seeming unbreakableness, his cleverness, perceptiveness, and bravery. A growing fascination with him. His intention to do well by this Jack.
He'd been a little blind to the Captain's potential back home. Maybe that had just been jealousy. Or maybe Jack needed a little encouragement to make them shine. He wanted to find out.
But right now, he'd bask in this one, giving him warm kisses.