They are caused by certain problems in brain chemistry. Often the brain does not have enough seratonin. Most antidepressants and St. John's Wort work by increasing the availability of seratonin in the brain. These disorders are inherited by the way of genes. Grief from a loss or a major tragedy can make someone very sad.
People can grieve for the loss of a loved one, job, home, health, mobility, and independence.
If the person is biologically prone to depression, this can increase the chances of developing the illness. Severe grief that lasts for more than a couple of months is probably depression and will improve with treatment. Those who experienced severe loss, tauma, neglect and abuse in childhood or violence at any age can also develop depression. It has been demonstrated that in addition to the psychological factors, these events can actually change brain chemistry. This is why medication is often so helpful.