The Hulk is comprised of a lot of things that Bruce keeps close, that he maybe isn't even aware of; he definitely doesn't know that the Hulk is anything more than the anger he's always felt bubbling under the surface. He is the loneliness Bruce felt as a child when his mother wasn't around, and later in high school when he didn't have friends so much as bullies and test tubes. He's the fear from that childhood too, and so many other things that Bruce's never known how to express.

The Hulk doesn't know how to express them either, or feel them; he just is them, and he now that he's being presented with comfort and stability, he wants to cling to it and never let it go, which is just what Tony wants him to do. That's what he needs to fill in all the vulnerable spots he just opened up, the glue to put himself back together. The kiss is surprising, obviously. It's the last thing he expects, but he's full of warmth and acceptance, and only some of it is Banner's.

"Okay, Tony," he says, and he looks down, needing a break from the intensity; it's almost too much for someone so unused to emotions beyond fear. When he lifts his head again, he thinks maybe he should give something Tony too, so he sets his hand on Tony's back and pulls him in closer in a kind of embrace.

"Thank you."