Please explain why people really have depression ?

Causes: There are multiple causes of depression including differences in brain chemistry, inherited traits, hormonal changes (i.e. Menopause, low thyroid), substance abuse and illnesses. High levels of stress, such as a divorce or history of abuse, can trigger depression. In some cases it can also be situational.

Combination: Biology plus life. We know that depression has a biological basis and has heritable genetics. We also know that life and your situation has an impact based on its events. Both issues singularly or in combination can cause depression.