Absolutely not! A diary is a very private thing and looking at someone's diary would be an infringement on their privacy. If they chose to share a part of it with me, that's another matter.
I had an experience when I was about ten where some kids broke the lock on my journal and I was totally insulted and hurt by it and vowed that it was the most terrible thing that could happen (of course, that is a child's way of thinking.) But, it stuck with me and I've always respected other people's privacy because of it.