The depressive state is a confusion of mood which is accompanied by sadness and by moral suffering. Depression is of variable intensity,  

It is not about a weakness of character or about a "lack of will". The mechanism of depression is not accurately known, but it is probably about a disturbance of chemical nature at the level of the brain, which draws away a biological fault of mood and of intellectual and physical functions. The reduction of the rate in the brain of a chemical molecule, the serotonin, and perhaps also other molecules, could play an important role.