As the summer passes, what is the cause of the animals' being happier than they ever thought they could be?(Chapter 3, Animal Farm)

After the Rebellion, the animals take upon themselves the
running and management of the Jones farm.  Inspired by the ideals of this rebellion,
Boxer works harder than he has ever done, and the others, with the exception of the cat
who disappears each day, work to their capacity.  Consequently, without the "worthless
parasitical human beings" to slow them down, the animals finish the harvest in two days
fewer than the time it has taken Jones and his men.  Furthermore, it is the largest
harvest ever as there is no wastage because the hens and ducks have gathered every last
stalk and no animal has stolen so much as a mouthful as they have
worked.

Having worked at the pace that they have chosen,
and having a deep personal investment in this work, the animals are happier than they
have ever thought possible: 

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Every mouthful of food was an acute positive
plesure, now that it was truly their own food, produced by themselves and for
themselves, not doled out to them by a grudging
master.

With the inspiration
of Boxer and the cleverness of the pigs who direct their efforts, the animals feel a
sense of satisfaction and pride heretofore unknown.