This is hardly surprising, since the novel is composed of a series of personal letters and journal entries. Supposedly, the writers of the letters and journals didn't intend them for anyone else's eyes as they were writing them. Some of the letters and journal entries include some pretty personal stuff:
There was something about them that made me uneasy, some longing and at
the same time some deadly fear. I felt in my heart a wicked, burning
desire that they would kiss me with those red lips. It is not good to
note this down, lest some day it should meet Mina's eyes and cause her
pain; but it is the truth. (3.29)
Um, yeah. Mina would probably not be too keen on her hubby-to-be getting all hot and bothered over some undead damsels. But it's snippets like this that give us the illusion that we're reading something super-private.
And that personal tone adds to the illusion of realism in the novel: The documents we're reading seem more legit because we can imagine real people (as opposed to fictional characters) writing them.