Since you are describing the events in chapters 1 through 3 of Animal Farm, it is important to remember that at the beginning of the book, when the farm was Manor Farm, the animals were working for the humans. They were slaves. However, after the rebellion they were working for themselves. They were free.
The first sentence of chapter 3 states:
How they toiled and sweated to get the hay in! But their efforts were rewarded, for the harvest was an even bigger success than they had hoped. (chapter 3)
There is a big difference between working hard because someone forces you, when you yourself gain nothing from it, and working hard for your own personal gain. We work harder for ourselves than for other people.
All through that summer the work of the farm went like clockwork. The animals were happy as they had never conceived it possible to be. Every mouthful of food was an acute positive pleasure, now that it was truly their own food, produced by themselves and for themselves, not doled out to them by a grudging master. (chapter 3)
Now that they are working for themselves, everything is better!