Your mind will take the shape of what you frequently hold in thought, for the human spirit is colored by such impressions.
—Marcus Aurelius, Meditations, 5.16
If you bend your body into a sitting position every day for a long enough period of time, the curvature of your spine changes. A doctor can tell from a radiograph (or an autopsy) whether someone sat at a desk for a living. If you shove your feet into tiny, narrow dress shoes each day, your feet begin to take on that form as well.
The same is true for our mind. If you hold a perpetually negative outlook, soon enough everything you encounter will seem negative. Close it off and you’ll become closed-minded. Color it with the wrong thoughts and your life will be dyed the same.
* Source: The Daily Stoic by Ryan Holiday and Stephen Hanselman