Your mind is like a computer; what you put into it is what will come out in your thoughts, and your thinking process reflects your perspective on life.
Don't believe me?
How about the Mayo Clinic?
Excerpted From Mayo Clinic -
Understanding positive thinking and self-talkPositive thinking doesn't mean that you keep your head in the sand and ignore life's less pleasant situations. Positive thinking just means that you approach the unpleasantness in a more positive and productive way. You think the best is going to happen, not the worst.
Positive thinking often starts with self-talk. Self-talk is the endless stream of unspoken thoughts that run through your head every day. These automatic thoughts can be positive or negative. Some of your self-talk comes from logic and reason. Other self-talk may arise from misconceptions that you create because of lack of information.
If the thoughts that run through your head are mostly negative, your outlook on life is more likely pessimistic. If your thoughts are mostly positive, you're likely an optimist — someone who practices positive thinking.
Simply put ...
If you change your thinking, you can change your life.