That is just wishful thinking.
We don't have power over many factors that affects us. To ilustrate, imagine a chronic ilness like some users reported, you can't simple cure it with willpower, positive thinking and hard work. "Oh, but the person could learn to live and be happy even with some ilness". Maybe, maybe not, humans are biological creatures with especific needs and limits. Chronic ilness is just the more evident example, there are so many others, more subtle things, in society, in the universe, in our bodies, that we have control over and that have the power the shape our lives, be it alone or together with other things.