Dadan is pretty much a mother to all 3 brothers, Ace, Sabo and Luffy since she “raised” them all. Garp entrusted her to take care of Ace shortly after the boy’s birth. Later, Garp also left Luffy in Dadan’s care. She took care of Sabo too when he moved in.
Dadan shows clear motherly love towards both Ace and Luffy and she got furious at Garp because he didn’t prevent Ace’s death.
Furious, Dadan accuses Garp of doing nothing while Ace died and berates him for putting his duty as the “Hero of the Marines” over the lives of his family. She attacks him with a wooden club, though Garp calls for his Marines to stand down as she is “an acquaintance of his.”
Dadan proceeds to assault Garp, throwing him to the ground and punching him in the face, while crying that he should die for what he did. Before she can attack him further, Makino intervenes, claiming that Garp is suffering the most for being unable to save his grandson. Dadan, however, replies that it is Luffy who is in the most pain. After imagining how awful Luffy’s pain must be, Dadan then vows to remain on Luffy’s side and shouts out loud her devotion to him.