Sam either didn’t want to tell Jessica about his upbringing and the family business, or he really did listen to his father in that regard. He seemed really pissed off that he was with Jessica a year and a half without telling her, and Dean was with “some chick” for a few dates and spilled everything.
Dean, as we’ve seen, follows John’s orders, but for Cassie, he broke them and told her what his family did for a living. Was it because he honestly loved her? From what Sam determined by Dean’s non-answers, I believe he did. Dean met Cassie during the time that Sam was at Stanford. He and John were finishing up a hunt in Athens, Ohio when he met Cassie. Was that separation from both John and Sam enough to send Dean looking for a substitute to latch onto?
Did John meet Cassie? We may never know that. I don’t think so, because Dean told Cassie he had to leave to help his father. So, either John was trusting Dean to do a job himself (which doesn’t seem like the case here), or even then, John was still leaving Dean (or the boys) behind while he did the supernatural hunting thing. John only called Dean when he needed help.
Obviously John grew to trust Dean enough to send him on hunts alone (New Orleans, ref: the pilot).
One other thing this shows: despite John’s pain and loss of Mary, Dean was capable of reaching out and loving a woman. After the womanizing we’d seen up until this point, the viewer might have had a hard time believing that Dean was capable of such deep love. Love of his family, yes, without a doubt. But to trust a woman with the family secret, to love a woman, after seeing what John went through? That had to take serious guts, especially to defy John in such a major way.