Imagine the following email:
"So, I was out last week with this guy and, NO, he wasn't a hermaphroditic dwarf with bad breath and a penchant for speaking in clichés!" Which surprises me and I think: "I wasn't thinking any of that! As a matter of fact, I had no preconceptions at all, Hector Projector!"
It's bullying. You're telling the reader that not only do you know how she thinks; you also think she's kind of stupid for thinking that way. Makes me crazier than the phrase "between you and I," which makes me CRAZY!
If I don't write it down it festers in the brainpan until I find myself driven to bad behavior.