We used to think of the human brain as changable & malluable up to a certain age but after this age was reached, it was assumed this flexibility in the brain was lost. Now, scientists are starting to see that adult brains are also changeable & malluable. This could have profound consequences, such as people breaking free from things that they have felt trapped by in the past, possibly people breaking free of addictions to emotions and addictions to other drugs.
When you are thinking about something new and novel, you are building new neural pathways in your brain (over time), if these new & novel ideas are thought of extensively, then connections between the neurons in the pathways will become stronger and stronger.
These new & novel thoughts (such as repeating to yourself over & over again ''I'm going to have a good day and be a positive influence on other people today'') will create neural pathways that become used instead of the older ones (such as ''Man, I don't want to go to work today, it's going to be stressful and unenjoyable''), if a neural network isn't used it will shrivel up.
Because in science nothing is experienced directly, something always has to be perceived, filtering through assumptions, beliefs systems, past experience until it comes together to form an individuals reality: because of this, changing the way you think, builds novel neural pathways, which in turn, gives another route for the perception of reality to travel through, resulting in a new reality for the individual. All by the power of thought.