For instance, take what's going on between Neville & Me.
He had a wet dream about me, which embarrassed him to the point at which he can no longer face me. I could comment on that all I wanted INSIDE his journal --though I haven't a clue what a girl should say to something like that. But all I can know OUTSIDE his journal (and thus, inside mine) is that he suddenly can't look at me anymore. Because those are actions that I have "seen" or "heard about".
So... in our own journals, we can only write about what we THINK or KNOW. And not about what's inside other journals.
I hope I haven't made all that too cunfusing...