Bah. This idea goes back in Western culture at least to the 1800s with William James, who, being a sufferer of depression himself, at least had the sense to know that there were only certain people for whom this sort of outlook to work, while others couldn't so blithely pretend that their problems weren't there. Recent interpretations of this theory have dropped that part, though.
But anyways... one of the main problems with the positivity theory in this case and similar ones is that it doesn't frame the solution in the right way. As it's presented, and has been elsewhere, believing that everything will go right for you --> success, and, conversely, therefore failure is the result of not believing enough. What this misses is the fact that there are extra steps between these two points. For getting from one to another, HOPE is necessary - not blind optimism. That hope, whether you believe/know the chance of getting what you want is .001% or 100%, will lead you to take the steps that might lead to getting it. The belief that it will just happen on its own if you think about it hard enough, however, is much more marketable, because it's both more definite and more lazy. Between a philosophy that says that yes, there is an OPTION of getting what you want, if you're willing to work for it, or one that says oh, if only you believe in getting it hard enough, it will come to you, which do you think most will subscribe to? In the end, it's the actions of someone that get them to success or failure, and their "half-full" or "half-empty" mentality may or may not have had a significant part in how they got there.
The other unfortunate thing about such theories is they have that nasty underlying message of "put up and shut up" for anyone who does have real problems that can't be solved with the Magical Power of Positive Thinking - or if they don't on their own, people often add it in, and therefore turn the "empowering" message on its head and make even more depressing for those unable to simply "turn that frown upside down" as easily as others tell them to.