It wasn't too horrible. At first.

[Until, you know...she found out why you weren't so hot with her.

Mary has the decency to look thoroughly chastised when Dean starts on about the journal. She wouldn't have done things differently if she had the chance, but she still feels a little bad that she pretty much read it behind his back.]

I had to know how your father raised and treated you...had to know if it was true.