Okay, so she has expressed her wants, so the next question is, what do you want? Do you want to be on the bottom, do you just not mind, or does it bother you? If you want to be on the bottom, then the best way to encourage her "topping" is to just keep on telling her how you love it, and that will give her a little more confidence each time. If you don't mind it, but it does nothing for you, then it's almost the same reaction- just keep telling her how you love her and want to help her feel happy and fulfilled. If it bothers you, then that's another story. She might be sensing that and that could be a reason she's getting upset.