"I am apt to fall into the Voice and manner of the Houyhnhnms"
When Gulliver returned from the lands where he felt superior to the inhabitants, Lilliput or Laputa, he readily readjusted to English life. When he returned from lands where he felt inferior to the inhabitants, he identified with
Gulliver with Houyhnhnms
them, and had trouble becoming an ordinary Englishman again. This happened after his return from Brobdingnag. It happens again upon his expulsion from the land of the Houyhnhnms.
His detestation of himself and his effort to become a Houyhnhnm were so intense and continued for so long, even after his return, that he had gone mad with frustration over his inability to transform himself into a completely rational horse, or in human terms, the completely passion-less, completely rational person.
Interestingly, this story-line of the struggle between emotions and rationality is still current with the character Spock in the television series Star Trek.