Genetic

According to the National Institute of Mental Health, there is strong evidence that depression runs in families. However, it is important to note that depression can also occur in people with no family history of depression. Certain combinations of genes along with environmental and other factors greatly increase a person's risk of developing a depressive illness.

Biochemical

Although scientists don't know for certain what causes depression, brain-imaging techniques have shown that those suffering with depression also show chemical imbalances. These chemicals, or neurotransmitters, include serotonin, norepinephrine and dopamine which regulate mood, sleep, aggression and other physiological functions.

Environmental

Sometimes extreme stress, the loss of a loved one, chronic illness, relationship problems or another trauma can trigger an episode of depression. This is even more common in people who are genetically vulnerable to depression.

Psychological

Psychological factors can also play a role in causing depression. People with low self-esteem, who have learned maladaptive coping techniques or who are easily stressed are more prone to depression than others.

Postpartum Depression

Many women experience depression after giving birth that is commonly referred to as postpartum depression. This is thought to be the result of the dramatic drop in estrogen and progesterone that occur after childbirth.

Source:

Mayo Clinic: Postpartum Depression

El Paso Mental Health: Biology of Depression--Neurotransmitters

National Institues of Health: Depression

More Information:

World Health Organization: Depression