I think that's wrong. Do you not think a 14 year old (for example) has the right to some privacy? What if she has thoughts she wants to put down but doesn't want you to know?
Speaking of the Princess Royal:
If she was sexually active I would certainly want to know so she could be put on birth control and be tested for sexual diseases.
If she was having suicidal thoughts or was depressed I would want to know to get treatment for her.
If she was very unhappy about school I would want to know so that the matter could be looked into and either handled or anothe school sought out.
If she was committing crimes or bullying someone I would want to know, but I don't know what exactly to do right now.
If she was having a problem with me I would want to know so I could examine my part in it and either change my behavior or be prepared when she brings it up.
In fact there isn't a single thing I can think of that I wouldn't want to know. Even if she were to write that she was transgendered I could prepare myself and her for the moment of revelation so I could be accepting rather than floored.
I suspect more people commit suicide because of what they're hiding than commiting suicide for sharing their hidden thoughts, fears, dreams, etc.