Renowned clinical psychologist Jordan Peterson said that if someone tell him that he is depressed he always ask them 5 questions: 1.Do you have a job? 2.Do you have a friend? 3.Do you have a romantic partner? 4.Do you have a serious physical illness? 5.Do you have a addiction? and if you answer on 3 of 5 with yes he can't help you because something of that problems will always again bring you down so first you should solve that problems. So external factors really matter a lot. Here is related video: http://www.youtube.com/watch?v=2c3m0tt5KcE&t=44m44s