It's a familiar sound that really tugs tight at his chest. He's familiar with this emotion. It isn't really all that pleasant, considering the large possibility that he's just going to get punched in the face and sent back out on his rear. Hence the fear.

And then he's chest-to-chest with Dean, and isn't made of stone like he used to be. He gets knocked back a foot or so, eyes wide and surprised like a deer caught in the headlights. His mouth opens- probably to offer some excuse or something, but nothing actually comes out. He does this for a second, mouth opening and closing with that vulnerable, startled look, like they're back in the brothel and Dean's ordering him a hooker.

"...Hello, Dean."