Been thinking about this and I remembered when I met my husband and we started a relationship he kept a journal and he showed to me the pages where he wrote about me. That was transparency I guess.
Me being nosy and alone in his flat one day I saw last years journal. Now he hadn't showed me that but I was nosey and at the stage where I wanted to know more about him. So uninvited I read his journal something I feel bad about to this day.
I found out a few things he hadn't told me which were deeply personal. One of which was that before he met me he really had the hots for my sister.

I have never been able to discuss the things I read because I should never have been reading them in the first place. It was over 20 years ago (no phones to check then!), it served me right I guess.
This incident pretty much formed my opinion on 'snooping' - I never did it again.
PS - It never did worry me that he had a crush on my sister because I was always secure it was me he wanted. I concede I am lucky like that.