Spike understands that feeling very well. He's always had a thing for violence, and seeing how she takes it is enough for him to stop holding back. Of course Buffy would be fine with it. It's fun.

But during the moment of hesitation that lets him make that decision, she gets her punch in. His head goes back with the hit, and he comes up grinning.

"And she's a living reminder of what happened to a good friend. Plus, I'm very much in love with someone else." With that he aims a sharp kick at her torso.