Steve2, thank you for your thoughtful contributions to this discussion. Your comments about genetic vs environmental factors are pertinent, especially in light of longitudinal studies of twins separated soon after birth. I do take exception to your overly general comments about schizophrenia since, in my experience and research, the consensus of the psychological community is that of increasingly acknowledging problems with the current ways the term is both defined by the APA and used in diagnoses and treatments of individuals presenting widely disparate behaviors. (See, for example: Schizophrenia Is Not An Illness, Philip Hickey, PH.D.)
But to return to the issues raised in the OP, one of the biggest questions among mental health professionals is this: What is the key causative factor which determines whether or not an individual develops a mental illness such as depression or anxiety?
Competing theories advance biological, environmental, psychological and emotional/social factors as primary. So which is it? The answer is: It's complicated. Indeed, all are contributing factors and yet none by themselves are solely determinant. Nevertheless, one is more prominent than the others. Which is it?
In 2013, in the biggest ever online test into stress undertaken by the BBC's Lab UK and the University of Liverpool, a study was done which revealed that rumination is the biggest predictor of the most common mental health problems in the UK. Although the study was originated in the UK, it revealed results that concern people worldwide, not just those in the UK.
This study, which was devised by the BBC's Lab UK and psychologists at the University of Liverpool, was the biggest study of its kind ever undertaken in the UK. It included a total of 32,827 people from 172 countries. Click on the link below for details:
In commenting on the results of this study, the BBC reported, “The findings of a ground-breaking study, published in the journal PLOS ONE, suggest that brooding [or ruminating] too much on negative events is the biggest predictor of depression and anxiety” and other associated symptoms of mental illness. - [Emphasis added]
In other words, “a person's psychological response” is a more important factor in determining whether or not they develop symptoms of mental illness than whatever specific traumatic events they have experienced. How we view an event is actually more important to and for our mental health that what actually happened to us.
This is good news, because this means that how a person copes, or fails to cope, with traumatic events is often the key factor as to whether or not they develop mental illness. People can be taught healthy coping skills to alleviate and even rid themselves of their depression and anxiety. One essential element is identifying and ridding themselves of dysfunctional core beliefs which contribute to their malaise and replacing them with a healthy, positive and accurate worldview which promote emotional health, personal growth, recovery and healing.