Quote from Davina McCall's 2005 novel "Yes Man" (made into the movie with Jim Carrie of the same title):
The sentence had tripped off the man on the bus's tongue like he'd been saying it all his life.
"Say Yes more," he'd said.
"Say Yes more," I'd repeated. Three little words of such power.
"The people without passion are the ones who always say no," he'd said, moments before, and I'd turned, stunned, to listen."
"But the happiest people are the ones who understand that good things occur when one allows them to."
And that was that.
That was all it took to turn my life on its head.
As the story develops, the protagonist takes this advice wholeheartedly and goes overboard, as a result, to find himself in hilarious situation after hilarious situation because he wants to say "yes" to nearly everyone, no matter the question for the remainder of the year.
However, the advice as given is not bad. In fact, it may be what a lot of people need to hear. (Not that there aren't people who are already saying "yes" too much who need the opposite advice of saying "no" more, which can be positive for such people.)