Depression, also known as depressive disorder, is caused by an imbalance of certain chemicals normally present in the brain that help control or initiate certain behaviors. Depression is distinctly different from feeling sad about an event or feeling "blue" for a short period. These feelings pass. Depression is an illness and needs treatment. It's not a weakness or something to feel ashamed about. Symptoms can be long lasting and debilitating and represent a change for most people from their normal experiences. One of the most significant changes is a loss of joy or pleasure from situations that once were fulfilling.