In what world would looking at anyone’s private journal be okay?

I can’t think of anything more invasive and cruel to do to a teenager than reading their diary. You really shouldn’t be reading anyone’s diary unless you’re trying to figure out the cause of their demise, ala Laura Palmer, or you’re reading diaries that were written to be eventually read by the public, ala Presidents. I guess you could argue the latter is true for Laura Palmer, too, but just roll with me. For the average Joe, your diary is the place where all of their most intimate private thoughts go: the thoughts that can be even too difficult to say out loud, so they must be written. Even if someone’s diary merely says what you ate for lunch that day, you shouldn’t look inside of it unless you’re comfortable with them getting incredibly upset. 

This applies to all kinds of notebooks and journals, even those that are not personal. Your coworker might have a «work journal» where they write all of their to-dos for the day, but who says that there isn’s something insane in that notebook as well? It might be a combination work notebook and dream journal, where they document that just last night they had a dream in which you threw them into quicksand? Do you really want to know exactly what your coworker is writing in their private notebook? If anything, I am sure that they don’t want you to know.