Our lives are full of setbacks and failures, large and small. Often the thing we dreamed of most, it no longer exists, and often the person we love most, can never return to us. Whenever these moments come, we experience sadness, pain, and even despair. Usually, depression and sadness caused by these clearly realistic events are normal, transient, and in some cases even beneficial to the growth of the individual. However, some people do not have very clear and reasonable external triggers for their depressive symptoms; other people, although some negative life events have occurred in their lives, have depressive symptoms that last for a very long time, far beyond the average person’s emotional response to these events, and the depressive symptoms worsen day by day, seriously affecting work, life, and school. If this is the case, then most likely, they are suffering from depression. On the other hand, it is not that a bad mood is depression. The diagnosis of depression has strict limits, whether it is the CCMD-3 diagnostic criteria in China, or the ICD-10 in Europe, or the DSM-IV in the United States, all have strict definitions of depression. China’s CCMD-3 has limited the diagnosis of depression from symptom criteria, severity criteria, disease duration criteria, and exclusion criteria, and only when the above four criteria are met can we call it depression.