From Dawn's end of things, it was an issue of opposites attract. She had the same sort of crush on Harvey that she'd once had on Spike, someone she shouldn't like but did, maybe a man of questionable morals. The bad boy. Even though Spike had been a lot more on the side of 'bad' back then than Harvey was... as far as she could tell. It was just a certain vibe he put off. Dawn was attracted to that vibe. And she really had no idea what to do with that attraction aside from continue to hang out with him.

Incidentally, she liked hanging out with him. Even when he infuriated her.

Like now. That 'I know' is so smug she could smack him. But it's also so cocky that it's the very reason she likes him. She could only ever wish to come off as cocky and self-assured as he does. And he makes it look effortless. She rolls her eyes in a most dramatic fashion. "Of course you know, all knowing Harvey. Of course you do."