Why does Gulliver decide never to lie again and never to return to human society?

Gulliver's decision not to lie is based on all of the faults of the people he has met and the true integrity of the Houyhnyms. His return to human society would mean that he would act more like the Yahoos who were incredibly savage and completely low in their behavior. Gulliver realizes how inhumane the real people that he has always associated with are so he never intends to live with humans again.