Depression can be the result of many factors, individually and acting in concert.
Environment
Reactions to events, often a loss in some form, are perhaps the most obvious causes. This loss may be obvious, such as the death of a loved one, or having moved from one house to another (mainly with children), or less obvious, such as disillusionment about one's career prospects. Monotonous environments can be depressing. A lack of control over one's environment can lead to feelings of helplessness. Domestic disputes and financial difficulties are common causes of a depressed mood. Love, or lack of being able to express your feelings can lead to a feeling of unexplainable sadness or grief.
Psychological Factors
Sometimes the depressed mood may relate more to internal processes or even be triggered by them. Pessimistic views of life or a lack of self-esteem can lead to depression. Illnesses and changes in cognition that occur in psychosis and dementia's, to name but two, can lead to depression. Depression may also be comorbid with cardiovascular disorders.
Physiological Considerations
A diathesis-stress model of depression (including clinical depression) is now widely accepted. This implies that underlying personality has some degree of influence over how the mood of individuals is affected by life events. The social, psychological and biological etiology of depression is still being actively investigated. The causal relationship with biological variables is unknown and so it is difficult to pinpoint the condition's roots. Some general physiological considerations include genetics (i.e. a hypothesized innate disposition to depression), neurochemistry (e.g. high levels of stress hormones such as cortisol, low dopamine activity), sleep patterns, female hormone imbalance (e.g. PMS in women), male hormone imbalance (testosterone) in men, use of medication (e.g. corticosteroids), chronic illness (e.g. diabetes or hypothyroidism), and seasonal factors (e.g. seasonal affective disorder related to hormones and sunlight).