Depression can be a very difficult and painful experience that affects not only the individual suffering from it, but also the people around them.
As a biological psychological factor, genetics play a big role. Mood disorders are inheritable for some people. Those that are susceptible to mood disorders can have an early age of onset, a greater rate of suffering other disorders in addition to depression and an increased risk of recurrent illnesses. However any genetic factors that are present must interact with environmental factors for depression to develope. Depression likely results from a combination of genetic, biochemical, environmental, and psychological factors.
Psychological factors that are extreneous accure with people who have been abused physically or sexually, children who have emotionally distant parent(s), lack of support, warmth, comfort, and love. Extreneous "present day" factors can relate to loneliness, unhappy working environment, or from a difficult divorce. With inner or inherent psychological factors, some personality traits are: excessive emotional sensitivity, excessive passive behavior, perfectionist attitude or personality, low or fluctuating levels of self-esteem, and pessimistic attitudes and thinking. People with some of these psychological factors present are more likely to develope depression because they are less able to cope with their family, work, and social environments. They tend to be overwhelmed by stress, tension, and problems where others are not.
With adolesents the developement of their brain does not fully happen until they reach the age of 25. The frontal Lobe is the last part of the Human brain to develope. The biological explination for rational behavior with adolescense is due to a specific region in the brain called the Amygdala which is responsible for instinctual reactions including fear and aggressive behavior. This