Because secrets do not increase in value if kept in a gore-ian lockbox, because one's past is either made useful or else mutates and becomes cancerous. We share things for the obvious reasons: it makes us feel un-alone, it spreads the weight over a larger area, it holds the possibility of making our share lighter. And it can work either way - not simply as a pain-relief device, but, in the case of not bad news but good, as a share-the-happy-things-I've-seen/lessons-I've-learned vehicle. Or as a tool for simple connectivity for its own sake, a testing of waters, a stab at engagement with a mass of strangers.
