After Whitebeard makes a risky gambit to draw off marine attention away from the Straw Hats, Luffy thanks him, and for the first time, calls him "Pops".
Whitebeard's reasoning for allying with the Straw Hats isn't simply because Ace asked him, but because Luffy is Ace's brother. So far as Whitebeard is concerned, the brother of his son is also his son.
Chapter 56: The Reveal that Merry has reincarnated into Sunny, and now has a soul that can fully interact with the crew. It drives half of the Straw Hats to Tears of Joy, and they immediately throw a party once they learn.
Chapter 64: Sanji finally regaining his past-self's memories. For the entire fic thus far, Luffy and Zoro have had to bear the burden of their own memories, believing that they had failed all of their friends. But with this, now do they not only know that they didn't fail (at least, not completely), but they also have another person who can share the burden with them and protect the rest of the crew.
