The Bible, St. Ignatius and Dr. Norman Vincent Peale are among the famous advocates of positive thinking. Wrote St. Paul:

Finally, brethren, whatsoever things are true, whatsoever things are honest, whatsoever things are just, whatsoever things are pure, whatsoever things are lovely, whatsoever things are of good report; if there be any virtue, and if there be any praise, think on these things.

Personally, I suggest you think on these things just before bedtime. It will do wonders for your attitude.
In philosophy William James creditably expatiated on this worthy spiritual insight. But there are men and women in lab coats who believe it also can be turned into a hard “science” — and hard cash. You can invest it with complicated mathematical equations and psychological jargon, then create “models” on computers, whereupon it is able, like a neodymium magnet, to attract nearly infinite grants of tax dollars and still more monies from slack-jawed private foundations. Who could argue with such a powerful theory?
Well, it turns out that for once a skeptical graduate student could. In England, Nick Brown made a cause out of debunking a paper by Barbara Fredrickson and Marcial Losada on “positive psychology” that was published in 2005 in American Psychology and has been quoted 350 times in other scientific literature. The result was a paper of his, with Harris Friedman and Alan Sokal in 2012, called “The Complex Dynamics of Wishful Thinking.” The story is told at Narratively by Vinnie Rotondano.
Positive psychology is essentially the science of misappropriations from serious science to dignify humbug.?I don’t know anything else about Narratively or Mr. Rotondano, but they have found the right angle — humor and persistence — to pry open the increasingly rusty box of contemporary peer-reviewed science. Other reporters won’t bother to read this, of course. Pity, that.
Meanwhile, if you are an interested professional, you can get a master’s degree in positive psychology at the University of Pennsylvania for $45,000. Alternatively, you can just sing to yourself Irving Berlin’s old song, “Count Your Blessings Instead of Sheep.” That last bit of advice, by the way, is my gift to you, free.