I find it interesting the development of the history of this. I believe it became a topic of interest first by conservative business people and salesman types and their Christian counter parts. A famous titles are: The power of positive thinking and Think and grow rich. Then you had cognitive psychology and then around the same time you got the new age law of attraction and positive psychology. Now you have Jill Bolte Taylor's book: My Stroke of Insight which is about as hard science as you can get on the topic. Also there is my website: upliftingthoughts.org as a possible contender. Anyway I feel like I sort of beginning to make some sense of all this. The main thing about positive thinking and negative thinking is I think they are kind of like light switch's in the brain. I believe that some switches seem to automatically want to go off into darkness and suffering because of a pattern of a persistent negative thought pattern that is expressed verbally or non verbally in what is really a thoughtless, mindless way. So if a person knows this then they can become conscious of this very easily if they can gather the wits to know that their brain is automatically playing the same negative tape over and over again. Incentive is that every time that play button is played the tape tends to get longer and longer and more and more of a drag for everyone and oneself. Incentive is that nobody wants to feel possessed like this. The joy comes fast when we are awake to these tapes and feel like we are looking down on them rather than them kind of looking down on us and bringing us down. That the best use of the talent of our left brain is learning not thinking. Also the other hemisphere of the brain which is the right hemisphere is the happy sister of it's left brain brother. Naturally when non language cues cause good feelings and some poetic language works the right brain is quick to say: Thank you, I am happy now. This sends a signal to the left brain at some point saying it is alright to let happy thoughts drift into your mind. Thoughts that are happy are cup more than half full thoughts, optimistic thoughts, constructing a better reality type of thoughts. When those kind of thoughts start activating they send a signal to the right brain as well. The left brain says: Enjoy the fruits of your positive thinking and have some fun, go out and enjoy nature or playing with your friends.
That's what I believe and I am sticking to it for the most part.