A person is always a biopsychosocial creature, meaning that there is a genetic component as well as psychosocial factors like life events, environmental stress, personality factor and coping strategies. There are some intresting views on depression explained by K. Abraham, M Klein and J Bowbly, but that would take us way off.
What we can conclude by all of this is, that the drugs he took induced the depression, or high psychosocial stress induced or some other illness induced it. Hey even an chronic inflammatoire disease may cause it. ( by activating the IDO system what may lead to tryptofaan depletion)
second i want to react to people who say it is hard to get hospitalised for it. I know from personal experience, few things are easier.
third, data retrieved from kaplan and saddock, 2000, say that depression occurs in 10-25 % of woman, 5-12%of men, while bipolar I 0,4-1,6 bipolar II 0,5 %