Dr. James Dobson addresses this concept in his book, Hide or Seek. In essence, children become what you tell them they are. If you tell her she's kind, she'll more than likely try to be kind. If you tell her she's a good girl, then she'll probably try to be a good girl. And, if you tell her she's a brat, she will prove you right.
Children take adults' words to heart.
They take our words and see themselves in that light. They will fulfill whatever role we give them. It's been said that a child's mind is like a bank - whatever you put in, you get back. We need to deposit appropriate, but true words into her bank. Of course, that doesn't mean you should tell her how good she is when she IS being a brat.