What does Pip say in his speech to Biddy and Joe? Why is his speech significant?

After Pip had recovered from this pleasant shock (Joe and Biddy are married) he begs Joe and Biddy to forgive him all his ungrateful behaviour and begs them both that in future when they have a son of their own they should not tell their son how thankless and ungenerous he has been. Both Joe and Biddly of course refuse to do just that:

`But I must say more. Dear Joe, I hope you will have children to love, and that some little fellow will sit in this chimney corner of a winter night, who may remind you of another little fellow gone out of it for ever. Don't tell him, Joe, that I was thankless; don't tell him, Biddy.

In Ch.59, Pip has decided to remain a bachelor for the rest of his life consequent to his failure of his 'expectations' in getting married either to Estella or Biddy and he begs Biddy to allow him to adopt their son Pip. Biddy refuses saying that he must get married and have children of his own:

`Biddy,' said I, when I talked with her after dinner, as her little girl lay sleeping in her lap, `you must give Pip to me, one of these days; or lend him, at all events.'`No, no,' said Biddy, gently. `You must marry.'`So Herbert and Clara say, but I don't think I shall, Biddy. I have so settled down in their home, that it's not at all likely. I am already quite an old bachelor.'