When Pip tells Biddy he wants to be a gentleman, she asks him, "But don't you think you are happier as you are?" Then she tells Pip that if he feels he has to change for someone, possible that someone isn't worth it. She's trying to tell him that a person who really cares for you would like you just the way and not expect you to try and be something you aren't. Pip notices that Biddy is really the opposite of Estella because she is always the same kind, caring person, not moody like Estella.
What Biddy knows that Pip doesn't yet realize is that money and social standing don't guarantee happiness. She knows it's the goodness in a person that counts.