What does Gregor do for a living? What does he dislike about his job?

It is revealed early in the novella that Gregor Samsa is a traveling salesman, and he loathes his occupation. He dislikes the isolation, repetitive nature of the work, and the draining effects of constantly traveling. He laments,

"Oh God, he thought, what an exhausting job I've picked out for myself! On the road day in, day out. It's much more irritating work than doing the actual business in the home office, and on top of that there's the trouble of constant traveling, of worrying about train connections, the bad food and irregular meals, casual acquaintances that are always new and never become intimate friends.''

Moreover, his boss hunts him down at home to confront him about his accused laziness. He is subject to constant scrutiny about work for which he cares very little.
The story details many specific gripes, but Gregor's malcontent essentially boils down to the following: "He was the boss's minion, without backbone or intelligence." He resents the dehumanizing effect his boss, his work, and his role as "the provider" have on him. It is so profoundly dehumanizing, Gregor is literally transformed into an insect, a "monstrous vermin." This makes the use of the word "backbone" especially poignant, as Gregor trades his literal backbone for the exoskeleton of a bug. His exterior finally reflects his interior state inflicted by his work life: "without backbone or intelligence."