Let's just say that Lady Booby doesn't like to be crossed. She's incredibly interested in Joseph, to the point that she'll do just about anything to sabotage his relationship with Fanny. She's also willing to totally go against class conventions in order to wed and bed her beloved Joseph. This is one lady who knows what she wants.
By the time Lady Booby's plot to send Fanny to the clink is foiled, she's completely "in the agonies of love, rage, and despair; nor could she conceal these boiling passions any longer, without bursting" (4.13.1). What gives? Well, for one thing, Joseph is the ultimate out-of-reach boy toy. Also, Lady Booby is used to her wealth opening doors. Joseph, on the other hand, doesn't care about wealth or fame—all he cares about is Fanny.