It's worth remembering the Japanese proverb: "All human affairs are like Saiou's horse," inspired by the story:
"Who Knows? The Farmer's Son: Fortune or Misfortune?"
One day in late summer, an old farmer was working in his field with his old sick horse. The farmer felt compassion for the horse and desired to lift its burden. So he let his horse loose to go to the mountains and live out the rest of its life.
Soon after, neighbors from the nearby village visited, offering their condolences and said, "What a shame. Now your only horse is gone. How unfortunate you are! You must be very sad. How will you live, work the land, and prosper?" The farmer replied: "Who knows? We shall see."
Two days later the old horse came back now rejuvenated after meandering in the mountainsides while eating the wild grasses. He came back with twelve new younger, healthy horses which followed the old horse into the corral.
Word got out in the village of the old farmer's good fortune and it wasn't long before people stopped by to congratulate the farmer on his good luck. "How fortunate you are!" they exclaimed. "You must be very happy!" Again, the farmer softly said, "Who knows? We shall see."
At daybreak on the next morning, the farmer's only son set off to attempt to train the new wild horses, but the farmer's son was thrown to the ground and broke his leg. One by one villagers arrived during the day to bemoan the farmer's latest misfortune. "Oh, what a tragedy! Your son won't be able to help you farm with a broken leg. You'll have to do all the work yourself. How will you survive? You must be very sad." they said. Calmly going about his usual business the farmer answered, "Who knows? We shall see."
Several days later a war broke out. The Emperor's men arrived in the village demanding that young men come with them to be conscripted into the Emperor's army. As it happened the farmer's son was deemed unfit because of his broken leg. "What very good fortune you have!!" the villagers exclaimed as their own young sons were marched away. "You must be very happy." "Who knows? We shall see!", replied the old farmer as he headed off to work his field alone.
As time went on the broken leg healed but the son was left with a slight limp. Again the neighbors came to pay their condolences. "Oh what bad luck. Too bad for you!" But the old farmer simply replied; "Who knows? We shall see."
As it turned out the other young village boys had died in the war and the old farmer and his son were the only able bodied men capable of working the village lands. The old farmer became wealthy and was very generous to the villagers. They said: "Oh how fortunate we are, you must be very happy", to which the old farmer replied, "Who knows? We shall see!"
So the moral of the story is: When something good or bad happens, you can never say for sure how it will turn out in the end. Some bad events turn out for the better, and some good events turn out for the worse. So it's best not to party too hard when things are going your way, nor beat yourself up too badly when they are not.
Adapted from: http://www.rainbowbody.com/newarticles/farmerson.htm