I am not aware of the exact origin of this story but I heard it from my mother and it has stayed with me all these years.
There was once a coastal town where the townspeople were dominantly fishermen who used to cast their nets in the waters to catch fish and sold it in the market to earn their living. They had been doing this for generations and gradually the fish population near the coast dwindled. This led them to decide that they need to go deeper into the seas to catch enough fish to subsist themselves. With time, they had also grown prosperous and had better boats and stronger nets to withhold the oceanic currents and hence they went off to their expeditions farther away from the coast. It improved matters and they started returning with bigger catches but they soon realized that the fish they caught did not taste the same as before. The customers who bought them complained that they tasted stale. The fishermen thought about it and figured out that earlier they used to catch fish near the coast and the time between the fishing and selling was relatively shorter. Now since they go farther from the coasts, by the time they return with the catch it didn’t remain fresh as before. So they all decided to carry large boxes of ice where they started storing the caught fish. Nonetheless, the customers still complained that the fish still would not taste as good as before. So it was decided that the fishermen would not kill the fish instantly but rather keep them alive till they reach the shore so that the dead fish would not go stale and tastes like the live fish off the coast. For this they carried large vats of water on their boats in which they dropped the fish caught in their nets. Now the fish remained alive till they reached the coast but, alas, the customers complained that something was missing and the fish would still not taste the same. This impelled a serious rethink among fishermen as this had started affecting their sales now. They observed that the fish in the large vats do not really swim around that much and hence they came up with a new idea. In the large vats, along with the small tasty fish they caught for food, they put a small shark. Due to the presence of the shark the small fish swam around furiously just as it did in the open ocean. Of course the shark would eat away some of the small fish but the fisherman still retained eighty percent of the catch. This is what they did and lo and behold, the customers said: wow, what fine fish you caught, it tastes just like before. And the fishermen were very happy.
Life is just like the small fish and the shark represents the various problems we face at different turns of our life. These problems keep us on our toes, in absence of which our life may lose its juice just like the small fish.