This question refers to Gulliver's last travels amongst the land of the Houyhnhnms and the Yahoos, and the way that he identifies so strongly and is so impressed with the way of living of the Houynhnms that he identifies his fellow humans with the yahoos, the uncivilised savages that the Houyhnhnms rule over. This means that when he is forced to re-enter human society, very much against his will, he finds it very difficult to slip back into his old life and ways of thinking. Note, for example, the following quotation in which Gulliver describes his fellow humans:
But, when I behold a Lump of Deformity, and Diseases both in Body and Mind, smitten with Pride, it immediately breaks all the Measures of my Patience; neither shall I ever be able to comprehend how such an Animal and such a Vice could tally together.
Gulliver's misanthropy at this point in the book is interesting for a number of reasons, and is clearly used by Swift as part of his overall project of satire. Gullliver holds up the example of the Houyhnhnms as an ideal to strive towards, however what this fails to acknonwledge is the way that there life and culture has profound problems to which Gulliver is completely blind. As much as the horses are clearly incredibly rational and virtuous, at the same time their lives are profoundly boring and lacking in interest. Swift deliberately presents their prosaic and monotonous life in contrast with the richness of human life and experience, with all of its many faults. Swift therefore points towards the way that Gulliver idolises a system of living that is impossible for humans to follow, and the way that he ceases to identify himself as a human. Swift implicitly criticises those who upheld nature as an ideal to be followed by humans and also those who would, like Gulliver, criticise humanity whilst being blind to their own hypocrisy.