These feelings don't go away on their own. They are not necessarily related to a particular life event.
Depression is a disorder of the brain.
Researchers believe that chemicals called neurotransmitters are involved in depression. Nerve impulses cause the release of neurotransmitters from one nerve cell to the next. This release allows cells to communicate with one another. Too little or too much of these important neurotransmitters may be released. This can cause or contribute to depression.