A bad mood or sadness that doesn’t go away can be sign of depression. This can happen to all kinds of people, no matter their age, gender, race, or how much money they have. Even though depression is common — especially in teens — some people develop it while others don't. Why?

What Causes Depression?

Lots of things influence whether a person has depression, such as:

Genes

Research shows that depression runs in families. Some people inherit genes that play a part in it. But not everyone who has a family member with depression will develop it, too. And many people with no family history still get it. So genes are one factor, but they aren't the only reason for depression.

Brain Chemistry

Certain chemicals called neurotransmitters (pronounced: nur-oh-TRANZ-mit-urs) manage mood. When a person has depression, these neurotransmitters might be in low supply or not be effective enough. Someone with the genes for depression may be more likely to have this neurotransmitter problem.

Stress, Health, and Hormones

Things like stress, drinking alcohol, using drugs, and hormone changes can affect mood and the brain's chemistry.

Some health conditions may also increase the likelihood of anger, irritability, or sadness. For example, low levels of thyroid hormones can cause a depressed mood in some people. Once a doctor finds and treats the health condition, these symptoms usually disappear.

Getting enough sleep and regular exercise often has a positive effect on neurotransmitter activity and mood and helps to prevent depression.

Daylight and Seasons

Daylight affects how the brain makes two neurotransmitters called melatonin and serotonin. They help control energy, mood, and when people fall asleep and wake up.

The shorter days and longer hours of darkness in fall and winter may cause the body to make more melatonin and less serotonin. This imbalance can lead to a type of depression known as seasonal affective disorder (SAD). Exposure to light may help improve mood in people with SAD.

Life Events

The death of a family member, friend, or pet sometimes leads to depression. Other difficult life events — like when parents divorce, separate, or remarry — can also trigger it. Whether or not tough life situations lead to depression can depend on how well a person can cope, stay positive, and get support.

Family and Social Environment

For some people, a negative, stressful, or unhappy family life can lead to depression. Other high-stress living situations — like poverty, homelessness, or violence — can lead to it, too. Dealing with bullying, harassment, or peer pressure also leaves some people feeling alone or anxious.

Situations like these don't always lead to depression, but facing them without relief or support can make it easier to be depressed.

Reacting to Life Situations

Life is full of ups and downs. Stress, hassles, and setbacks happen (but hopefully not too often). How we react to life's struggles matters a lot. A person's outlook can contribute to depression or it can help guard against it.

Research shows that a positive outlook helps protect against depression. This is true even for people who have the genes, brain chemistry, or life situations that put them at risk for developing it.

The opposite is also true: People who tend to think more negatively may be more at risk for developing depression.

Making an effort to be optimistic — like believing there's a way around any problem — helps ward off depression. So does developing coping skills and a support system of positive relationships. These things help build resilience (the quality that helps people bounce back and do well, even in tough situations).

How Can I Be More Resilient?

Here are 3 ways to build resilience:

1. Try thinking of change as a challenging and normal part of life. When a problem crops up, take action to solve it.
2. Remind yourself that setbacks and problems are temporary and solvable.
3. Build a support system. Ask friends and family for help (or just a shoulder to cry on) when you need it. Offer to help when they need it. This kind of give and take creates strong relationships that help people weather life's storms. Some people also find comfort in their place of religious or spiritual worship.

Being positive and resilient isn't a magic shield that automatically protects us from depression. But these qualities can help offset some things that might lead to the condition.

What Else Should I Know About Depression?

Depression is mental health condition that requires medical care. If a sad or bad mood lasts for weeks, months, or even longer, call your doctor, who can connect you with a therapist.

Therapy helps people get extra support during tough times. By talking with a therapist, they learn to build new skills, cope with their problems, and feel better.