For me, I think that the interesting thing about the whole situation is that as soon as Faith wakes up her initial reaction is to go to the high school and straight back to the only person she felt showed her love, he father figure. She was going to return to him, not seek Buffy out to exact revenge right away. Who knows what she would have done if she still had the Mayor when she woke up. In her dream sequences she had her 'dad' with her, having a picnic and its a geniune smile and genuine contentment, only to have Buffy come through and attack her. She's paranoid of Buffy taking that love away from her, maybe she would have went back to her and stayed away from Buffy awhile in fear of her taking the mayor away.
But when she wakes up, the only person that opened their arms and took her in, in her eyes, is gone. Everything she had build up, is gone. And the people that did that are all still here and getting on with their lives. I think this fuels Faith's reaction to go back and see what Buffy is up to, in Sunnydale she really doesn't have anywhere else to go.
I can completely understand in her eyes why she wants revenge, not that I condone it or think she is in the right to believe that. But she feels left behind, the only other person in the world like her moving on and shes stuck alone and with nothing, her first reaction is going to be to lash out, and Buffy seems like the obvious choice. I think when Faith goes to Joyce and talks to her, we really see all the insecurity coming out and the jealousy she has for Buffy, she wants to be her, have her friends and her family, so the body switch is just kind of ironic, Faith learns more about herself in Buffy's body and what it is to be a slayer than she ever did in her own skin.
Okay thats kind of sidetracking, but I can see why she wanted revenge, and can see why Buffy wouldn't want to remember her, Faith wasn't exactly the nicest person in Season 3
