That is one of the things I admired about Spike - he was the only person who totally accepted Buffy as the Slayer! I can see why - in season 7 - he was the only one she saw as guarding her back. He never doubted that Buffy was right, that Buffy was capable, and that Buffy knew what she was doing.

Angel tried to help her, but he really started to see her as someone he had to protect and had to make decisions for - and she went along.