It's not what happens to you, but how you react to it. A person behaves differently depending on whether his expectations are positive or negative.
An example with a non-person subject: Say you are extending your hand to pet a dog. If the dog has negative expectations (thinks you are threatening and possibly going to hurt him) he will shrink away or try to bite you; if he has positive expectations, (thinks you are going to pet him or give him a treat) he will act friendly or at least not run away. The dog's first reaction would cause most people to become hostile toward him; if he does bite you, your automatic defense reaction will cause you to fight back. The dog's second reaction can engender affection, or reinforce your previously held friendly feelings. The dog's expectations profoundly influenced his behaviors; not always, but often, negative behaviors engender negative outcomes, and positive behaviors engender positive outcomes.
If you believe that something will turn out well, you are likely to react to circumstances in such a way that you will fulfill your own prophecy. Positive (or negative) beliefs influence a person's reactions, so that he often make his own expectation come true. The concept of the self-fulfilling prophecy is based on the idea that a person's reactions cause a previously-false belief to become true.