Depression is a mood disorder characterized by sadness, a loss of interest in normally pleasurable activities, or a feeling of loss that interfere with a person’s daily life. We all experience these feelings once in a while, but if they persist for a long time, it may be a sign of depression. The symptoms need to go on for at least two weeks to be considered depression.
Sadness is a symptom of depression, but it is not the same thing as depression. Depression can last weeks, months, or years, affecting a person’s relationships, ability to work, and their physical health condition. Not everyone experiences depression the same way.
There are many different types of depression distinguished by how they manifest. Some of the most known types are major depression, persistent depressive disorder, postpartum depression, and psychotic depression. Postpartum depression is seen in women, usually following childbirth.
According to the World Health Organization (WHO), depression is the leading cause of disability among adults and adolescents worldwide.  Data published by the Center for Disease Control and Prevention (CDC) estimates that about 8.1 percent of American adults aged 20 or more had depression in any given two-week period between 2013 and 2016. 
Depression does not arise from a single cause. In fact, scientists are not aware of what causes it, but many suggest it happens due to a combination of different factors. Some of them are discussed in this article.
Stressful Life Events
Stressful life events such as the loss of a job, the death of a loved one, or a failing relationship can affect people in very significant ways. People cope with things differently. Not everyone who experiences any of these things will end up having depression. Many of them don’t, but the impact of such events which can have on some people is life-changing.
The body has a physiological response to stress. Often, this involves altering the chemicals in the brain. In most people, the brain’s chemistry returns to normal sometime after the stress has passed. In some, the disturbance can be more enduring. Changes in a person’s brain chemistry can have a significant effect on their mood.  This may present as depression in some people. The process is complex and not yet fully understood.