Diaries should always be kept secret.
The purpose of diaries and secrets, from my point of view. A diary is probably the most "secret place" in the entire world; because you're keeping your own thoughts, etc., all to yourself, until you decide otherwise, which brings me back to (auto) biographies/memoirs, as mentioned previously. The secrecy within diaries and the human creative expression is connected to intimacy, and it is vital since you own and write in this book daily, and no one else knows about it but you. This is why a diary is one way in which you can creatively express yourself freely without noise or interference from anyone or anything. You don't have to be a seasoned professional author to do this exercise in your own place, and on your own time, time being one essential factor that we lose on a daily basis getting older. People have been known to buy locks and/or vaults for their personal diaries, this gives a further sense of privacy and security, like nothing else in our lives really can....