She's jealous.

Willow looked at Buffy, hurt in her eyes. "Buffy?"

"Yeah, Will?"

"Why didn't you have me do that to you?"

Buffy stroked Willow's cheek. "I pushed Joan into it because I was proving a point to her. But you..." Buffy smiled. "When you do that, I want it to be because you ask to. I want you to want it as much as I do."