Why was Gulliver impressed by Houyhnhnms ?

Gulliver has come to love the Houyhnhnms, their society, and their way of living. He writes, "I had not been a Year in this Country, before I contracted such a Love and Veneration for the Inhabitants, that I entered on a firm resolution never to return to human Kind, but to pass the rest of my Life among these admirable Houyhnhnms in the Contemplation and practice of every Virtue."