Perhaps Jennifer doesn’t realize that she feels guilty for the way she threw Daniel over when Jack came back. Danniel was hers to have, and she didn’t want him. Now Jack is gone, and she cannot accept that Daniel has moved on without her. (I guess she doesn’t realize that this is a pattern for hero Dan, he always gets involved with his female patients). Jen’s hatred for Nicole has esculates every time she sees her with Daniel.
Abby’s issues of abandonment date back to Jack always leaving her. Next, she couldn’t commit to Cameron, and he moved on. Then her "best friend" leaves without explanation. All these losses have piled up, and she reacts.
Both of them need counseling big time, and to stay out of others’ business. They are turning into Maggie big time.