Human beings belong to higher animals, so they have emotions, and all emotions actually have a material basis, such as the substances secreted by the brain, including 5-hydroxytryptamine and norepinephrine. When exercising or being happy, 5-hydroxytryptamine and norepinephrine will be secreted, and a feeling of happiness will occur, and when their secretion is insufficient, an unhappy feeling may occur, which can be referred to as the phenomenon of depression. Depression may sometimes be transient or persistent. Transient depression, such as being caught late for work by the boss and being criticized or fined, may appear as depression for a few days, but if one suddenly receives praise or a raise in salary, one immediately shows happiness again, which is then not known as a depressive illness, but as a transient depressive state. If the depression persists for more than 3 months, the possibility of depression should be considered. Depression is a disease that requires systematic treatment in a specialized hospital. The phenomenon of depression is very normal and as changeable as the weather, but it is not a disease state if it can be eliminated in a short period of time. As long as you are not suffering from a disease, you are a healthy person and do not have to feel depressed.