We do not know what exactly causes clinical depression for any particular person. There are many theories about causes including biological and genetic factors, environmental influences, and childhood or developmental events. It is generally believed that in most cases it is often caused by the influence of more than just one or two of these factors. Working with a therapist is the recommended way for an individual to initially learn about the root causes of their particular depression. Regardless of the cause, remember that the majority of depression is treatable.