Depression can have many different causes.

There are so many different factors that play a role ~ and factors vary from person to person.

That said, there are many common symptoms of depression.
Depression symptoms include:

• Feelings of sadness or unhappiness
• Irritability or frustration, even over small matters
• Loss of interest or pleasure in normal activities
• Reduced sex drive
• Insomnia or excessive sleeping
• Changes in appetite — depression often causes decreased appetite and weight loss, but in some people it causes increased cravings for food and weight gain
• Agitation or restlessness — for example, pacing, hand-wringing or an inability to sit still
• Irritability or angry outbursts
• Slowed thinking, speaking or body movements
• Indecisiveness, distractibility and decreased concentration
• Fatigue, tiredness and loss of energy — even small tasks may seem to require a lot of effort
• Feelings of worthlessness or guilt, fixating on past failures or blaming yourself when things aren't going right
• Trouble thinking, concentrating, making decisions and remembering things
• Frequent thoughts of death, dying or suicide
• Crying spells for no apparent reason
• Unexplained physical problems, such as back pain or headaches

For some people, depression symptoms are so severe that it's obvious something isn't right. Other people feel generally miserable or unhappy without really knowing why.
Depression affects each person in different ways, so symptoms caused by depression vary from person to person. Inherited traits, age, gender and cultural background all play a role in how depression may affect you.
Depression symptoms in children and teens
Common symptoms of depression can be a little different in children and teens than they are in adults.

• In younger children, symptoms of depression may include sadness, irritability, hopelessness and worry.
• Symptoms in adolescents and teens may include anxiety, anger and avoidance of social interaction.
• Changes in thinking and sleep are common signs of depression in adolescents and adults but are not as common in younger children.
• In children and teens, depression often occurs along with behavior problems and other mental health conditions, such as anxiety or attention-deficit/hyperactivity disorder (ADHD).
• Schoolwork may suffer in children who are depressed.

Depression symptoms in older adults
Depression is not a normal part of growing older, and most seniors feel satisfied with their lives. However, depression can and does occur in older adults. Unfortunately, it often goes undiagnosed and untreated. Many adults with depression feel reluctant to seek help when they're feeling down.

• In older adults, depression may go undiagnosed because symptoms — for example, fatigue, loss of appetite, sleep problems or loss of interest in sex — may seem to be caused by other illnesses.
• Older adults with depression may have less obvious symptoms. They may feel dissatisfied with life in general, bored, helpless or worthless. They may always want to stay at home, rather than going out to socialize or doing new things.
• Suicidal thinking or feelings in older adults is a sign of serious depression that should never be taken lightly, especially in men. Of all people with depression, older adult men are at the highest risk of suicide.

When to see a doctor
If you feel depressed, make an appointment to see your doctor as soon as you can. Depression symptoms may not get better on their own — and depression may get worse if it isn't treated. Untreated depression can lead to other mental and physical health problems or problems in other areas of your life. Feelings of depression can also lead to suicide.
If you're reluctant to seek treatment, talk to a friend or loved one, a health care professional, a faith leader, or someone else you trust.
If you have suicidal thoughts
If you or someone you know is having suicidal thoughts, get help right away. Here are some steps you can take:

• Contact a family member or friend.
• Seek help from your doctor, a mental health provider or other health care professional.
• Call a suicide hot line number — in the United States, you can reach the toll-free, 24-hour hot line of the National Suicide Prevention Lifeline at 800-273-8255 to talk to a trained counselor.
• Contact a minister, spiritual leader or someone in your faith community.

When to get emergency help
If you think you may hurt yourself or attempt suicide, call 911 or your local emergency number immediately. If you have a loved one who has harmed himself or herself, or is seriously considering doing so, make sure someone stays with that person. Take him or her to the hospital or call for emergency help.

Cause

It's not known exactly what causes depression. As with many mental illnesses, it appears a variety of factors may be involved. These include:

• Biological differences. People with depression appear to have physical changes in their brains. The significance of these changes is still uncertain, but may eventually help pinpoint causes.
• Neurotransmitters. These naturally occurring brain chemicals linked to mood are thought to play a direct role in depression.
• Hormones. Changes in the body's balance of hormones may be involved in causing or triggering depression. Hormone changes can result from thyroid problems, menopause or a number of other conditions.
• Inherited traits. Depression is more common in people whose biological family members also have this condition. Researchers are trying to find genes that may be involved in causing depression.
• Life events. Certain events, such as the death or loss of a loved one, financial problems, and high stress, can trigger depression in some people.
• Early childhood trauma. Traumatic events during childhood, such as abuse or loss of a parent, may cause permanent changes in the brain that make you more susceptible to depression.

http://www.mayoclinic.com/health/depression