The "Fantasy Realisation Theory"

It is speculated that people confuse daydreams for reality. The warm emotions from the fantasies lead them to feel as if they'd already met their goals. It results in not putting in the hard work needed for success.

Positive thinking on its own could be counter-productive. Research shows that dieters who fantasized about weight loss are less likely to lose weight. Students who dream of academic success tend to get worse grades than those who don't.