Spike: That's my girl.
Buffy: I am not your girl! You don't have a soul! There is nothing good or clean in you. You are dead inside!
Thoughts? What do you think Buffy reacted the way she did to Spike saying that?
She was already ashamed of her "relationship" with Spike. His trying to make it into something more than just twisted, violent sex just made what she was doing (using him to hide from her other shame - that she was hating her friends) that much worse.
I think it was much easier for her when he was acting sleazy. When he started to show any sign of being 'person-like', it made it more difficult for her to treat him like an object and that forced on her uncomfortable questions like a) What the hell am I doing? b) Why can't I stop this? c) Do I have feelings for him - can I have feelings for anyone, anymore? d) Why can't I get over what they (Willow, etc) did to me?