Buffy was staring daggers at Willow for bringing her to her house, her place of sanctuary and safety (for the most part).
"Come on Buffy, you know we need to have all the muscle we can get for this big bad." Willow said, her pleading eyes searching for Buffy to understand. Willow loved her best friend, but sometimes she was just too stubborn. Buffy had her arms crossed and then let out a sigh. She turned towards Faith and said, "Do anything, and I mean anything, that might jeopardize the safety of this house and the people in it, and you're out." Faith, who was leaning against the door frame, said lazily "B, I'm no stranger to your rules." She then walked into the kitchen to visit with the new Slayers.
"Buffy…" Willow started.
"No, can it Will, I understand, I'm just not…" Buffy trailed away and she began to get lost in her thoughts when Willow nervously asked, "Your just not what Buffy?" Buffy didn't know what to say. She knew what she was going to say. She was going to say she wasn't happy Faith was here, but she couldn't say it.
"I just don't want to deal with it." And with that she walked up the stairs to her room, Willow having no idea that Buffy's reason for not wanting Faith there was because the it, was the same it she had when she was 17 and Faith popped up as Kendra's replacement. Willow might be thinking that she hated Faith, well everyone did, but she didn't. Buffy only wished Faith would have the decency to ask for help and that she, Buffy, could save her. She understood the younger slayer way better than any. She understood her fall to evil when she killed that man, because she had to deal with the same thing. She knew if she didn't have Xander, Willow, or Giles that she might've taken the same road when she killed Scott. FUCK, how she wished she could turn back time and give Faith what she needed then, a friend. And of course, there was also the unknown motive she never figured out until she switched bodies with Faith. When she looked in the mirror (when there was no one there), she saw something. That something Buffy wanted to see everyday, Faith's face. Unhindered by the real owners walls, it was then that Buffy realized how her teenage naivety led her away from a possibility of happiness. She remembered all those late night stake outs with the younger of the pair and the realization that Faith made her happy the way Angel did scared her. So when Faith went over to the Mayor it became easier and easier to push those feelings away.