I believe it is She knows that he has made the only decision that will satisfy his soul but im not completely positive.
It shows an evolution which will alienate him from society for the rest of his life. Indeed, Gulliver has found a race of talking horses that have the perfect society with no crime or vices or any form of corruption. These race of Houyhnhnms is the master of the island and rules over savage, dirty and grotesque humanoid creatures which are called the Yahoos. After spending sometime in the island, Gulliver feels more Houyhnhnm than human and considers humans a backward species. It is to be noted nonetheless, that Gulliver is rescued by a Portuguese captain that is very generous and sensitive and his refusal and disgust to acknowledge this implies that he still has the vice of pride very much ingrained in his nature.
