A depressive disorder is not a passing blue mood. It is not a sign of personal weakness or a condition that can be willed or wished away. Without treatment, symptoms can last for weeks, months, or years. Bipolar depression includes periods of high or mania. Not everyone who is depressed or manic experiences every symptom.
Some people experience a few symptoms, some many.
Also, severity of symptoms varies with individuals. In reading the above paragraph, specifically the last line, one can understand just some of the difficulties in dealing with depression. During times of stress, or if there is a trigger event, such as job loss, or death of a loved one, your brain will cease to create this chemical. This allows you to feel depressed. But if one doesn't come to terms with these stresses, over time, your brain will forget how to create this chemical.