Self-fulfilling prophecies are another example of how what we believe influences how we experience reality. According to wisegeek.com a self-fulfilling prophecy is "a statement that alters actions and therefore comes true." For example, a person who goes to a party thinking, "I'll be miserable. No one will talk to me." might appear to others as standoffish or seem so unpleasant that people at the party are reluctant to approach him. Thus, the person's actions (he was defensive because he believed no one would talk to him) brought about what he predicted would happen - he appeared so unapproachable that no one came over to talk to him.
One common type of self-fulfilling prophecy involves what we tell ourselves about what we need in order to be happy. If we say, "I can't be happy if I'm not wealthy, successful, respected, loved, etc." or "I can't be happy until I have a college degree, get a good job, get married, etc." then it is very likely that we will not be happy until we have or achieve these things. I've known many people who either consciously or subconsciously tell themselves, "I can't be happy alone" or "I can't be happy if I'm not in a relationship." They then feel compelled to seek out a relationship, even if it's with the wrong person, because they believe they need it in order to be happy.
It is worth asking ourselves -- what am I telling myself I need in order to be happy? Are there people who don't have these things who are happy? What's the difference between me and them? Is it what we are telling ourselves? Is it how we think about the situation? Is it the attitude we've adopted? Can we learn to tell ourselves that we already have everything we need in order to be happy?