Depression causes distress and prevents people from functioning at their peak. In general, studies have found that people with depression experience a lower quality of life in a number of areas:

• Poorer physical health
• Lower levels of psychological well-being
• Poorer relationships with family and friends.

They also show:

• Higher levels of marital discord
• Poorer work performance records
• More on-the-job problems
• Missing more days of work due to health-related issues
• Poorer interpersonal relationships
• Poorer psychological well-being
• Less satisfaction with their relationships
• Lower marital satisfaction
• Less intimacy
• More physical and emotional pain
• Lower self-esteem

This is a fairly complex question with a complex answer; moreover, science hasn’t totally answered the question yet. The current theory is that multiple factors combine to affect mood and the brain systems associated with depression. Some of these factors increase your vulnerability to depression (risk factors), and some protect against depression. Both risk factors and protective factors can be biological, psychological, or social/environmental in nature. You will sometimes hear this described as a biopsychosocial model of depression

Current thinking also views risk factors as a kind of underlying predisposition to depression. For instance, if you have a family history of depression, you may have a genetic predisposition to depressive problems. However, this does not mean you are destined to have depression or any other disorder. Typically, the predisposition has to be paired with current life stressors in order to trigger depressive symptoms. This is called the vulnerability/stress model of depression.

Not all stress is unhealthy. Humans actually need a certain amount of stress to function effectively. Good stress (called eustress) provides energy and motivation to surmount obstacles. Most of us face a multitude of stressful situations each day and navigate them successfully. However, when stress becomes too intense or is unrelenting, it becomes distress. This type of stress is damaging and can lead to health problems, such as heart disease or ulcers, as well as the depressive symptoms - fatigue, inability to concentrate, guilt, and feelings of hopelessness. Recent studies suggest that chronic stress precedes most (as many as 80 percent) of the episodes of depression.

A study done by researchers at Temple University found that not only does stress lead to depressive symptoms, but depressive symptoms increase your exposure to stress. This reciprocal relationship between stress and depressive symptoms suggests that becoming more depressed actively contributes to generating additional stress that, in turn, increases your risk for future depression. This interaction creates a cycle of ever-increasing negative emotions, thoughts, and behaviors.

In order to determine whether a medical condition may be causing your depressive symptoms you should review the order in which symptoms appeared. If the depressive symptoms started or worsened around the same time that medical symptoms became apparent, then it’s possible that the depression symptoms may be due to an underlying medical condition.

A second consideration is the way the symptoms of depression change when the medical condition is treated. If the mood symptoms improve when the medical condition is treated, it is more likely that the medical condition caused the mood disorder. If the mood symptoms don’t improve, then the depression symptoms are probably not related to the medical condition.

Besides physically causing depressive symptoms, medical conditions may also psychologically lead to these symptoms. The burden of having a disease or condition can contribute to the stress, frustration, sadness, or hopelessness that a person experiences. Having a serious medical condition can also lead to anxiety, causing insomnia, weight loss, and confusion.

Abusing substances, including marijuana, alcohol, cocaine, and amphetamines, can also lead to depression. The negative effects of substance abuse on personal relationships, health, finances, and behavior can cause secondary depression, which tends to improve as the substance abuse is controlled or, in the case of dependence, treated.

However, substance abuse can also lead to primary depression because of the effect of substances on neurotransmitters in the brain. Note: some people may use substances such as marijuana or alcohol in an attempt to self-medicate the symptoms of depression. They mistakenly believe that such substances will make them feel better. While these attempts at self-medication may provide some very short-term relief, in the long run, they can actually exacerbate depressive symptoms because of the negative effects of substance abuse on people’s lives.

Shame, like guilt, is considered to be a “self-conscious” emotion. That is, it develops during childhood in response to social interactions rather than coming prepackaged (as do the emotions of fear, anger, sadness, and joy) in the neural hardware of the brain. An early experience of shame can lead to significant loss of self-esteem and a pattern of underachievement as the person avoids trying new activities or learning new skills in which they could disappoint themselves or be judged negatively by others. The feelings of personal inadequacy created through shame can also be a risk factor for developing depressive symptoms, and can affect the way you organize and make sense of your current and future relationships.

Although shame and guilt are often viewed as interchangeable, there are several important distinctions between these two emotions. Guilt is an emotional response to an action that either hurt another person or violated a social norm; shame is an emotional response to a public or private exposure of personal inadequacies, which may be exaggerated or entirely manufactured. While guilt involves negative self-judgement of a behavior, shame involves negative self-judgement of the whole self. You can understand a fundamental difference between the two experiences this way: guilt says, “You made a mistake” whereas shame says, “You are a mistake.” Shame has significantly stronger associations with depressive symptoms than guilt does.

If you believe that you or someone you love might be perfectionistic, here are some questions to consider:

• If you don’t complete a task perfectly, do you feel like a failure?
• Are you not satisfied until every little detail of a project or task is dealt with?
• Do people tell you that you’re too much of a perfectionist?
• Did your parents punish you when you made a mistake?
• Do you feel that people will think less of you if you make a mistake?
• Even when you’ve done something well, do you feel your performance wasn’t good enough?
• Do you take an unusually long time to complete tasks to make sure they’re done “right”?
• Do you feel that you never quite live up to other people’s expectations?

Perfectionism is defined as the tendency to set demanding goals and standards for oneself and to believe that failing to achieve these goals is a sign of personal unworthiness. Perfectionism can be a positive thing if it motivates you to do better work and to try harder to achieve your goals. However, it can also be a negative trait that is associated with shame and depression.

If shame is part of your depressive profile, you likely feel dominated by your “inner critic,” the internalized voice of those experiences that taught you to feel unworthy. If you have perfectionistic tendencies, this is the voice that punishes you for not meeting unrealistic standards and goals.

Certain patterns of thinking have been associated with depression. These patterns are called “cognitive distortions” or “cognitive biases.” Here are descriptions of some of the most common distortions in thinking.

All-or-nothing thinking (dichotomous thinking): IF you use all-or-nothing thinking when making sense of events, you tend to experience things in black-and-white terms. You many see situations as either entirely “good” or “bad” or view people as either “right” or “wrong” when there’s actually a middle ground you aren’t considering. You may be putting your thoughts into the “either/or” format. For example, you may decide that a friend is “bad” because he made an insensitive comment about something that is important to you (“either Jeff supports what I say, or he’s not someone I can count on”).

Exaggeration: Exaggeration is sometimes called “catastrophizing” because when people use this form of thinking, they blow the imagined consequences of an event way out of proportion. They see events and situations as more dire or important than they really are. For example, you might think that getting stuck in traffic is going to completely ruin your day. If you’re stuck in this type of thought pattern, you may get accused of “making a mountain out of a molehill.” Using words and phrases like “disastrous,” “horrible,” and “this ruins everything” in your conversation or self-talk may be a sign that you are using exaggeration or catastrophizing.

Overgeneralization: This type of thinking is extremely common. You overgeneralize when you take one isolated experience or piece of information and draw serious, and usually bad, conclusions from your experience. In this way, isolated negative events are viewed as a never-ending pattern of negative events, horror, or defeat. For example, you might interpret a setback in your training as “I am just no good at it and will never get better.” Or you might interpret a flat tire as “the car is always falling apart.” Using words such as “always” and “never” in your conversation or self-talk are signs that you are overgeneralizing.

Mind-reading: In this type of thinking, you assume you know what another person is thinking (or will think), even though you have no evidence to back up your assumptions. When you’re mind-reading, you almost always assume that what someone is thinking about you is negative. For example, if you assume no one wants to hear about what you went through while you were deployed, you are using the negative thinking pattern of mind-reading. Communicating is the only way you will actually find out what anyone else is thinking...and it may well be more positive than you assume. Self-talk that includes “she probably thinks…” and “he doesn’t understand…” are signs that you may be using the negative thinking pattern of mind-reading.

Minimization: If you use minimization when making sense of events, you tend to shrink the importance of some events that deserve far more weight than you give them. For example, you may minimize or ignore your successes or personal achievements, or you may skip over your good qualities and only focus on your (or others’) weaknesses. You may dwell on the negative events in your life and forget the things that are going well.

Emotional reasoning: If you draw conclusions based on how you feel, you are using emotional reasoning. That is, you interpret your emotions as evidence of some general truth. For example, you may notice that you feel anxious as you are doing some job and infer that there is a definite and true cause for your feelings. You decide that it’s dangerous, so you start scanning the environment for threats instead of recognizing that your feelings are a result of how you interpreted your feelings.