There is no standard “starting point” or cause of depression. The causes of depression vary from person to person. It could happen for various reasons and can be triggered by numerous things, depending on the person who has it. Some people can become depressed because of stressful and unpleasant events in their life, such as the loss of a loved one, divorce, redundancy, illness, or financial problems.
This is not an exhaustive list of possible causes, as there are many more reasons why people could get depressed. You would have to speak to a mental health professional to pinpoint the cause of your depression and get appropriate treatment.