Motives:
Being a  sweetheart as she is, she takes pity for Joe. After Mrs. Joe passed away, she wasn't at the forge all the time, but she was there often enough. 

She and Joe eventually fall in love and are married. Still, there is always that little spark of jealousy and envy towards Estella. Who could blame her, for she is only human and the complete opposite of Estella.

This is the way that a heartfelt conversation was ended by Biddy. Pip and Biddy had had some good times, and now that he was going to become a gentleman, Pip wanted her to help Joe along. Although Pip didn't mean to be offensive, Biddy sensed that he meant something different from what he said.