I've wondered about this, and obviously part of it comes from her horrible mother. Fanny is such a thoroughly nasty piece of work though, that there must be more to it. I wish that JA had told us!

HOWEVER, since JA didn't, I found at the JA Society of North America's (JASNA) website a very interesting backstory, written by Harriet Walter, who played Fanny in S&S2. Director Ang Lee had asked the actors to develop answers to a set of questions regarding their characters, and Miss Walter took that ball and ran with it! She read her story at a JASNA-NY meeting on July 10, 2009, and it was printed in the JASNA online magazine, Persuasions, which I've linked below.