They might be afraid of being judged. For example, if they have had some difficult experiences in their past, they don't want others to look down on them. So they choose to hide their life story.
Sure. In a forest at night (the darkness), there could be all sorts of animal behaviors and interactions that we don't know about. There might be a story of how a certain small animal is trying to find food or shelter among the larger predators. We can't see these things clearly in the dark, so the stories remain hidden.
Well, sometimes it's obvious when they get defensive or change the subject quickly. Also, if their story doesn't add up or has contradictions, that could be a sign.
Well, 'darkness hides story' might imply that difficult or unpleasant situations (represented by darkness) conceal a narrative. Maybe it's about how bad times in history, like wars or disasters (which are dark periods), have stories of heroism or survival that are not always visible at first glance.