Why does Gulliver reject the job?

I assume that you are referring to book 1, chapter 5 of Gulliver's Travels. In this part of the story, Gulliver, using his enormous size, drags the whole Blefuscudian fleet to the shore of Lilliput, preventing a full-scale invasion. The Lilliputian Emperor is suitably impressed with Gulliver's heroic endeavors and promptly awards him with "Nardac," Lilliput's highest honor. But the emperor is not finished with Gulliver just yet. Having seen what he is capable of, the emperor wants to use the giant Gulliver to finish the job; he wants him to go back to Blefuscu and destroy their entire fleet. The Blefuscudians won't just be defeated, they'll be utterly vanquished, so much so that they will be turned into nothing more than a colony if the emperor gets his way.
But Gulliver is not interested. He will have no part of a plan that will turn the Blefuscudians into slaves. The emperor will never forgive Gulliver for his refusal to destroy Lilliput's most hated and despised enemy. So he and his court lackeys start plotting the best way to bring about Gulliver's downfall.