From Professor Gin Malhi's talk above:
Stress, pressure and underlying causes of depression and anxiety.
What may be okay for one person, may trigger an event in another person.
For example - we all have immunity at different levels - if we are exposed to the same bugs, the person with lower levels of immunity will get the infection. That's not to say that the other person wouldn't get the infection if they're exposed long enough.
The level of immunity that we start off with, will be a gene determinant.
Our brains are functioning in terms of our immune system and our stress axis - it's all unique to each individual. You can't compare and say this person didn't respond to this loss with depression. Why did it affect this person in such a drastic manner?
Our brains develop and evolve over 20-25 years. During that time, they become unique organs. Each have strengths and weaknesses. In terms of attributes that function perfectly well, and some that have some scarring/some vulnerability, and that may well be because of the early environment that the person has grown up in.
In terms of emotional disorders and what causes depression, there are a number of factors.
There is a genetic component - we know it runs in families.
There's also a strong hormonal component because its much more common in women and girls. The role of stress is pivotal - low grade chronic stress, rather than acute stress is much more damaging.
Stress does damage braincells and kill braincells. That is going to be a factor in developing anxiety disorders as well as depression.