I agree with WisNeuro. Some common themes in personal statements are fascination with the human mind, desire to help people, experiences with people with mental disorders, etc. Most of us identify with these themes in some way. None of them are verboten in a personal statement, but try to stay closer to the present and to those experiences and interests that show more clearly that you know what you're getting into. One of your aims here is to persuade the reader that it is worthwhile to train you as a psychological scientist and practitioner.