Dean's a bit bemused by the verbal torrent; he may not fully understand what the hell Buffy's talking about, but living the life he and Sammy have, he thinks he can relate at least a little. Sometimes, destiny hands you the short end of the stick.

And then expects you to bury it in the nearest monster.

"Yeah, Buffy. Anything you need, we'll help." No, he doesn't know her, but Sam apparently does, and thinks enough of her to make that kind of offer--of course he's in.

He focuses his gaze back on his brother, shaking his head. "Dude. From my point of view? Nothing happened. You were here. You've been here this whole time, only in the two minutes it took me to walk out and get a Coke, you materialized a girl and a story about being in another city. I haven't been anywhere else. Do you know what did this to you?"