Expectations.

I've found that whatever you expect, with confidence, becomes your own self-fulfilling prophecy. 

When you confidently expect good things to happen, good things usually happen to you. If you expect something negative to happen, you are usually not disappointed. 

Your expectations have an inordinate effect on the people around you as well. What you expect from people and situations determines your attitude toward them more than any other factor, and people reflect your attitude right back at you, like a mirror, whether positive or negative.