Thinking positive creates a positive reality:
We tend to think that our thoughts are meaningless and have no effect on reality. In truth however, our thoughts help shape reality, and influences it to go in the direction of the thought. This is why it is so important to think positive. When one is challenged with a worry in a certain matter, thinking positive actually helps shape the positive result. The same applies towards speech and action, when one speaks positive and performs actions that are positive it helps shape that the end result will be positive in reality.
 Likkutei Dibburim 1/159; Igros Kodesh Rayatz 2/537 in name of Tzemach Tzedek; Mamar 1988 “Habaim Yashreish Yaakov [printed in Melukat 4/130]