KiKi in Cali,
In this instance I was referring more to negative thoughts and assumptions.
Often, the negative thought patterns and core assumptions we hold of ourselves and our world lead us further down the negative spiral.
If we can track those negative thoughts and figure out what the underlying issue is we can begin working on stopping that downward spiral as oppose to feeding into it.
In regards to your question, I have recopied a past post of mine explaining what some of the causes of MDD are, happy reading!
The exact causes of MDD are unknown, although biological, psychological, and social factors all likely play important roles in the illness:
Genes
In some families, MDD affects someone in every generation, suggesting a biological (genetic) component to the disorder. However, MDD also happens to people with no family history of depression. So, in some cases, a vulnerability to MDD may be inherited and in other cases not. Genes are probably not the whole story. Whether inherited or not, MDD, and especially chronic MDD has been associated with some fairly specific changes in brain chemistry and structures.
Stress
Stressful life events such as the death of a loved one, relationship problems, and marital problems, problems at work, financial problems, or unemployment can trigger a depressive episode. Sometimes even positive life changes such as marriage, new relationship, job promotion, or moving can trigger a depressive episode.
The brain changes in MDD may be related to the brain's response to stress (or bad experiences) in early life. Later in life, this may cause the brain to react to one or more new negative events. Without any negative life events, people may avoid the risk for developing depression.
Neurotransmiters
Neurotransmitters are chemicals that convey or inhibit communication between nerve cells. Research has implicated a number of transmitter systems in MDD including serotonin, norepinephrine and dopamine.
Brain Structures
The use of modern brain imaging techniques has shown brain structure changes associated with MDD in a number of brain areas (including the limbic, orbital and prefrontal cortex, amygdala, ventral striatum, and hippocampus).
Psychological Factors and Personality
Some people with certain characteristics seem to be more likely to suffer from MDD. Some of these personality types include: people with low self-esteem, people who are pessimistic, those who self-criticize, perfectionists, and people who are easily overwhelmed by stress. These characteristics sometimes represent a psychological predisposition to depression. Sometimes they represent early symptoms of MDD.
Other Factors
Other events associated with MDD include childbirth, menopause and seasonal changes.
Medical illnesses
Stroke, heart attack, cancer, Parkinson's disease, infectious diseases such as influenza and hepatitis, and some hormonal disorders can cause depression. Depression complicates these medical illnesses because the depression makes people less able to look after themselves properly and work towards recovery and getting back to health.
Medications
Some medications such as antihypertensive medication, oral contraceptives and corticosteroids can cause depressive symptoms.
Most often it's likely that some combinations of biological, psychological, social and environmental factors are involved in the onset of an initial episode of MDD. Once a person has had one episode of MDD, new stressful events can trigger future episodes. While most people can identify a "trigger" for a depressive episode, it is sometimes difficult or impossible to identify an event or stress. Sometimes depression seems to just happen out of nowhere and come "out of the blue."