The Hulk smiles, slow and surprised. Not at the sentiment, because he's heard Tony reassure him like that before, but at the kiss. And not even that it happened because Tony's kissed him before too, but because it warms something in him too. It doesn't actually take a lot of thought or buildup for him to decide to touch Tony's cheek again and then to kiss him, really kiss him. The idea that Tony might not want it doesn't even cross his mind; he's that convinced of Tony's acceptance of him.