Rating: R to be safe
Faith thinks she and Buffy could never be friends. They talked about it once, after the Potentials threw Buffy out of the house and elected Faith leader in her place.
Faith suggested, then, that it was because there were never supposed to be two Slayers. She doesn't believe that anymore. Now, she thinks they exist to balance each other: dark and light... once, good and evil. And the price of that is never being the sisters they should have been.
No, they'll never be friends.
But when Buffy is writhing beneath her, crying her name, that doesn't matter at all.