If you know someone who is depressed, please resolve never to ask them why.
Depression isn't a straightforward response to a bad situation.
Depression just is.. Like weather.
Try to understand the blackness, lethargy, hopelessness and loneliness they're going through. Be there for them when they come through the otherside.
It is hard to be a friend to someone who's depressed, but it is one of the kindest, noblest and best things you will ever do.