People seem to have this impression that Han Solo wants to run and gun his way through everything. It's mostly true when you get him behind the controls of a vehicle. Sometimes true when you can observe him in person. The thing of it is, he rushes through a lot of things so that he can take his time with others.

Behind closed doors with another good-looking pilot is something he doesn't want to rush.

The hands in the hair is among the ultimate expressions of trust that Han can give to someone. It's not something he lets people just do ... but for some reason, he's alright with Carol doing it. (In the same way, perhaps, that she doesn't really let people use endearments apart from Han.) It's taken as an invitation to further explore. You touch me and I touch you back.

He lets his hands drop lower still. To the point where you couldn't really say they were on her 'back' anymore and he traces over curves that he was simply admiring before.