You are the Placebo, Chapter 3.

The Placebo Effects on the Brain.
If you are thinking the same thoughts as yesterday, you are more than likely making the same choices today, leading to the same behaviors, producing the same experiences, and creating the same personal reality.
When we repeat a thought enough times, our brain cells strengthen and increase the number of connections between each other. When you think new thoughts, you gain new neural connections. As we repeat the thought, we strengthen the connection.