Why? "we are what we think"

It is true, "we are what we think" and here is a golf analogy to illustrate.
I once played for the golf championship of the world. Ok, not the whole world but my two man team had won the Wednesday night league and we played the Thursday night league winners for the grand championship. When we came to the last hole and I figured we were one down. It was a par 5. I blasted a super drive, hit a nice three wood to 30 yards and pitched to 12 feet.  With complete focus I drained the putt for a birdie. My partner hit a nice 6 footer for par and the opposing team congratulated us for the win. You see, I had figured wrong. We went into the last hole not one down but tied. I have always remembered that small moment because of how my thinking affected how I approached that hole.  I believe if I had known we were tied I would have been less aggressive and would have tried to get nothing worse than a par. I thought we were behind, so I went all out. Yes the Championship of the world, won because I "thought" we were behind.  A small moment but a larger lesson, "we are what we think" and what we think makes us who we are. You can think your way to misery in your life or to happiness in your life. I choose happiness.