Reading someone's diary is a major invasion of privacy and truly never okay. However, that doesn't stop people from doing it.
There is a gnawing curiosity that is often triggered by the sight of someone else's unattended diary, and too many succumb to the temptation. While some diaries are littered with in-depth descriptions of meals, or long-winded accounts of work days, others are full of secrets not meant for the prying eyes of the outside world.
My wife and I found my 9 year old nephew's diary once. All that was inside, written in crayon, was "daddy's fat". We still laugh about it.
I read one of my mom's old journals from when she first got married. Turns out she got pregnant with my older brother because she was trying to save their already rocky marriage, it hadn't even been a year yet. I'm the second of five children and my parents have always had a bad relationship. They're still together, and not very happily honestly, at almost thirty years in. Makes me wonder if all five of us were marriage band-aid attempts.