Buffy stands before him, eyes glazed with tears, hand pressed to her mouth in shock and the urge to hit her again makes the world roar in his ears like a jet engine breaking the sound barrier. He doesn't need her attempting to make him feel guilty when she's the one in the wrong. Instead he carefully speaks, using a tone of voice he hasn't used on her since those first few weeks in Sunnydale. Then it was because he was trying to keep his muddled emotions in check. Now it's because if doesn't say something using that detached, bored, ironic tone he's afraid of what might tumble out of his mouth.
She however doesn't pull her punches, she never has and her snotty anger enrages him because all she's doing is assuming that he's too stupid to handle Faith. It's nice to know that she has zero confidence in him. He can smell her boyfriend all over her and that just makes it worse. She's moved on without a second thought.
He's about to say something he can't take back after all; she's pushed him past the point of caring when Wes interrupts. There are more important things to worry about. Bitch floods his mind. She's a self centered bitch. He turns a little and catches a glimpse of her unguarded face. She's terrified for him. And worried that his feelings for her have evaporated. This time the guilt and sorrow don't disappear.