They really aren't comfortable with each other, and I mostly blame that on Buffy. She's always been The Slayer, and she doesn't know what to do when "The" becomes "a." So she takes that out on the new Slayer. I'm fairly sure she did the same thing to a lesser degree with Kendra, too. And Faith reacts to that defense mechanism with her own, and conflict grows exponentially. Faith wants to be liked, she just doesn't think she deserves it. Buffy unintentionally uses that against her, which leads to her switching to the dark side.

But if they could ever get past all of that, they'd be so amazing.