When Dean put an arm around her, he sealed it for her. He wasn't interested. There was something strictly platonic about the way he was touching her. And teased her.
The last thing the world needs is me, she thought but didn't say. What good was she most of the time? Yeah, today she'd helped Dean but even that had been selfish. If he'd been someone she couldn't care less about, he'd have been on his own completely and she'd have booked it the hell out of there. There was no way she'd have stuck around for just anybody out of the goodness of her heart and she knew it. Dean might have been built that way, as he said - which in and of itself was a bit of a crush to her ego, even if she'd never admit it - but Shannon sure as hell wasn't.
"Oh, I'm hot now?" she teased back, rather than let him see what she was thinking. She nudged him gently with her elbow, not wanting to hurt him. And with that gesture, up went the walls again. High, strong, and opaque. She was good at that and he'd be none the wiser. What he didn't know wouldn't hurt him and what she pretended didn't exist wouldn't hurt her, either.