Even with my dairies from my mid-20s to late 40s, I worry about discovery. There's something very vulnerable and scary about your deepest thoughts and feelings being exposed to someone else who not only wouldn't understand, but might even judge you.
I used to only half joke with a longtime friend that I would want her to come over to my place and gut it of all my journals if anything happened to me...the thought of my parents, or anyone really, reading years' worth of words that only mean something to me leaves me feeling bad about myself, if that makes sense. My longtime friend (of twenty five years) knows everything bad there is to know about me so I guess I wouldn't worry about her as much as I would anyone else.
On the other hand, though, I think of a line from an old Three's Company where Cindy explains that her mother found her personal diary and read it from the very first page to the last. When asked what happened, Cindy says her mother fell asleep because it was the most boring thing she had ever read.
Because I struggle with nightmares so much I do keep a dedicated dream diary. It helps a little bit, but I write in that more because my dream life is so much more interesting, no matter how troubling.