Could you tell me what are signs of severe depression and what leads to it?

Depression etiology: I agree with dr. Stewart's response. Simply put, causes are; bio psycho social. There can be sleep/appetiter/energy changes, sadness, thoughts of self harm, helplessness/hopelessness, & loss of interest, all affecting daily functioning.

Lack of Interest: To be diagnosed with major depression, for at least two weeks, one experiences a depressed mood and/or loss of interest in previously pleasurable activities. Other required symptoms are disturbances in sleep, appetite, and concentration, excessive guilt, lack of energy, hopelessness, restlessness and recurring thoughts of death or suicide. Contact your md as effective treatment is available.