I have found that most people have good intentions and are concerned and want to help and that is why they ask. Over the years I have become very open about it. If I tell them the truth that "I dunno it just came out of the blue and hit me like a ton of bricks" they don't find that satisfactory because they are looking for a simple answer that can be easily fixed. To them there has to be some simple cause. This often opens an argument or discussion on the nature of depression. I have had many of them but it is usually not helpful to me. I am just trying to help them understand and it is ground I have covered a million times. If someone sincerely asks out of concern I will answer honestly though and try to avoid the big discussion unless I am in the mood to have it. I don't harbor any ill will if they don't understand or agree. It is risky because I don't want to feel judged or add to the shame I already feel.