My son doesn't keep a diary -- but I do have the password to his FB, although I haven't used it in years. And I routinely check the history of his computer. (Which, btw, I just checked yesterday and found more porn than you would see at a pimps & hos convention).
My youngest son brings his too me all the time. My oldest I respect his journal.
I have encouraged them to write journals since they were 2 years old. They see me writing in one all the time. We set aside time 2-3 times a week to write in our journals. Even if it's only our favorite poem, what happened today, a drawing, etc.
The only way I would snoop into my kids journals/diaries without them knowing is if I suspected something.
No. No matter what, every person has a right to private thoughts and opinions. Reading a teens diary is so invasive. They need a place where they can say anything without fear of being judged, ridiculed or get in trouble. A diary is a great place to let feelings out a get a better understanding of things in one's life. Now, let me clarify, if my child was going to hell in a hand basket and I felt that he/she was in serious truble, then yes, I would read.