Conversation so far:
How can you tell if someone is suffering from depression?
You may be depressed if, for more than two weeks, you've felt sad, down or miserable most of the time, or have lost interest or pleasure in usual activities, and have also experienced several of the signs and symptoms across at least three of the categories below.
What are common types of depression?
he four most common types of depression are major depression, persistent depressive disorder(formerly known as dysthymia), bipolar disorder, and seasonal affective disorder.

Question: What causes it
Self-contained rewrite: What causes depression?