The causes of depression are complex and differ from person to person. Among other causes, childhood trauma, abuse, bereavement, an unstable upbringing, significant life events, divorce, mental or physical health problems, anxiety, eating disorders, post-traumatic stress disorder (PTSD), substance abuse, etc 

When people are clinically depressed, they do not think clearly. They hold a pessimistic view of the future, unrealistic expectations, and are unusually harsh and overly critical of themselves.