Buffy stopped when Spike tugged on her sleeve, but she still couldn't manage to look him in the eyes. So, there they were just two people who loved each other, but couldn't look each other in the eye. Spike's love was different though. He loved her. He obsessed over her. She wasn't sure he could actually exist without following her around. Buffy's love was much more complicated. She had a hard time reconciling her feelings for Spike with this image of herself and she was ashamed of the way she felt the majority of the time. If she wasn't feeling guilty over loving Spike and keeping it a secret from her friends then she was feeling guilty over rejecting someone who had quite literally given everything he had just to prove something to her, just to save her. It was a whole lot of guilt that Buffy didn't know where to put or what to do with. Kissing Spike wasn't going to solve any of it. It would probably make it worse, but they had tried to give up on each other. Hell, Spike had died in the process and yet here they were again. Maybe the universe was telling her something.
"Everything's already changed," Buffy admitted, but the admittance gave her the confidence to look Spike in the face. "We're just dancing around the subject, but the fact is you died. You died to save me because it was the last thing you could give to me. The only thing that would really matter and you did it." And it made me love you. "You were gone and now we're both here and I don't know what to do with all of that. You deserve someone who will treat you so much better than I ever can. I don't understand why you can't just let me go. I know why I'm here, Spike. I just don't know why you are."