He smiles warmly at her, eyes half-lidded as his gaze falls to her lips, then back up to her eyes.

"Good. I guess that's how you can be sure you'll never lose me again."

Said with a hint of humor but also a lot of truth to it, even if he doesn't mean she actually needs to put a tag on him or anything. She won't lose him again, it's what he means by that - he won't ever leave, even if he might get the urge to just as desperately as he did after Sokovia.