Worsening of persons mood may not be triggered by any external element. It's hard to determine its endogenous origin. Mostly is sufferer first confronted with life events that might trigger depressive condition, and just when no particular source of the mood disorder is found, the depression is considered endogenous.
The treatment, however, is basically identical with treatment of non-endogenous depression, with exception of necessity of non-pharmaceutic therapy, which is meaningless, if roots of the depression are endogenous. Although psychotherapy isn't usually prescribed, knowledge of techniques to deal with stress and anxiety may be beneficial to the patient.