Why does Pip tell Biddy about his attraction to Estella?

Pip told this to Biddy because in this chapter, he notices that she is happier and more settled, and even a bit pretty. He realizes that she has been a good friend to him, a good friend and teacher, and he recognize clearly the difference between her and the cruel, cold and beautiful Estella. He realizes, when they are at the table one night, that "perhaps I had not been sufficiently grateful to Biddy," and he resolves to make her more of a friend, and to treat her better. She had done a lot for him, teaching him, caring for his sister and Joe, and he felt grateful for it. So, he asks her for a walk and a talk the next week.