OptimalHappiness
In Buddhism, there is a concept called impermanence, which teaches us that nothing is permanent, and we should treat our lives as such.
On o positive side, our problems are impermanent, meaning that if today we feel bad tomorrow (or a week, month, ... after) we will be able to feel good again eventually!
On a negative side, the good things in our lives will pass, too. As such, we should not cling to things hoping that everything will last forever, as we are just preparing for one big disappointment.
Besides Buddhism, many other religions and folk stories are talking about this same concept in many different ways. Perhaps, my favorite story is by 19th-century retelling of Persian fable written by an English poet Edward FitzGerald. He writes: