A parable from a Chinese text called the Huainanzi, which was written sometime before 139 BCE (the text containing this parable was presented as a gift to the then-Emperor Wu of Han that year, so it was presumably finished sometime thereabouts), goes something like this:
“The old man of the frontier lost his horse,” but “how could he know if this is not fortuitous?”
This parable has been rephrased over the generations, and in its modern incarnation it’s usually presented as:
“The old man lost his horse, but it all turned out for the best.”
The moral of this parable is that while it’s obviously not great to lose one’s horse, especially if one lives in a frontier-land where a horse is presumably quite useful, such a loss may not be pure misfortune and may even turn out to be better than the alternative.
Maybe, had the man not lost his horse, he would have gone for a ride, fallen off, and broke his back.
Maybe the horse would have taken him along a path where he would have been beset by highwaymen.
Maybe had he not been stranded at home due to a lack of suitable transportation, he would have gone into town, and consequently missed out on a wonderful, memorable, life-defining day with his family.
The message, then, is that while we might interpret things as bad or good in the moment—favorable or unfavorable based on what we’re assuming about future events (and thus, planning around)—we actually don’t have any real sense as to how things will turn out.
It may be more beneficial, then (especially in circumstances in which there’s no profit to fixating on the downsides of a situation), to interpret things differently, perhaps even deciding to believe that a misfortune may result in something positive.
This concept has in recent years been reinvented for the social media world, often presented as “burnt toast theory,” which posits pretty much the same concept but through the lens of a more modern malady: you’re getting ready for work and you burn your toast. That error forces you to make new toast, which in turn means you leave home for work later than intended.
You can’t know that the burnt toast (and consequent delay in leaving for work) didn’t save your life, as it’s possible that you could have been in a car accident or struck by a meteorite had you left home on time as intended.
Of course, focusing on alternative, theoretical (and positive) possibilities is arguably just as irrational as focusing on downsides.
But part of why this concept is sometimes promoted within self-help circles is that we seemingly have the choice to feel down and pessimistic after we suffer burnt toast (or the metaphorical equivalent), or to instead focus on a theoretical reality in which we dodged something even worse (or will enjoy something great) as a consequence of that non-ideal thing that happened—and this chosen perspective can, at times, spark (or help sustain) a more upbeat, growth-focused mindset.
There hasn’t been any solid research looking into the possibility that “lost horse” thinking might itself be beneficial, but there has been research into positive thinking more broadly, and results have generally shown that replacing pessimistic thinking with any excuse for optimism will tend to be a net-positive, as long as you still learn lessons, when they’re available to learn, from the bad things that happen in your life.
It’s possible to swing too far in the other direction, ignoring important, negatively biased information in favor of positive interpretations of everything, which can disconnect us from reality and prevent us from learning those aforementioned lessons.
But for most of us, most of the time, it seems like putting a positive spin on negative happenings can help us maintain a more consistent can-do attitude, while also helping assuage possible sources of stress and anxiety before they influence our moods and behaviors.
This puts me in mind of "The Bridge of San Luis Rey", a 1927 novel by American writer Thornton Wilder. The premise, from Wikipedia: "The Bridge of San Luis Rey tells the story of several interrelated people who die in the collapse of an Inca rope bridge in Peru, and the events that [led] up to their being on the bridge. A friar who witnesses the accident then goes about inquiring into the lives of the victims, seeking some sort of cosmic answer to the question of why each had to die."
The wikipedia article, at https://en.wikipedia.org/wiki/The_Bridge_of_San_Luis_Rey , is insistently positive about the book. But I loosely recall a Marxist critic in the 30s savaging it as a kind of quietist or right-wing propaganda.