For as he thinks in his heart, so is he.
Proverbs 23:7 (NKJV)
In John C. Maxwell's Winning with People, the Lens Principle says that who you are determines what you see. This includes how you see yourself, how you see others, and how you view life and the world around you. And finally, who you are determines what you do in life. And who you are is determined by five things: genetics, self-image, experiences in life, attitude and choices about those experiences, and friends. Anyone of these can be a positive or negative influence in your life.