Any neurotransmitters imbalance in the brain can cause the mood to go down. This makes the person depressed. This can happen due to genetic impact. The social or financial difficulty can cause the disturbances in neurotransmitters, which causes depression in a person. Try to know from him whether he has any history of failures which he is hiding within him. Try to know the exact cause of depression. Depression, if left unattended, could develop in to Bipolar Disorder.