The man who becomes deeply depressed when his business is not successful is using money as a love object. A known territory - Humans, like animals, need to feel inside a known and trusted environment, so people when they move tend to become depressed. A creative outlet is essential to a human. Parents who have lived their lives through their children tend to become depressed when children leave home. Unless we feel we are of value to and loved by others, we cannot love ourselves, e.g., the depression of the unemployed is not due just to the shortage of money, but of self-esteem.
Some people are more sensitive to small stimuli.
Some show a cyclical pattern of depressive attacks. In a few cases they are suffering from a potentially severe illness called manic-depressive psychosis, in which bouts of wild overactivity alternate with severe depression. Depression is like all illnesses, self-limiting. Depression like all acute illnesses will get better by itself, in most cases, in time. Depression affects people in different ways.