What was one of the strongest motivations for completing the rebuilding of the windmill?
It is also important to remember that Animal Farm is an allegorical tale for the Russian Revolution. Therefore, the windmills are representative of the Russian 5 year plans. Each plan was developed to increase economic development.
So the windmill was orginally planned to have a 3 day work week. Of course at the end of the book it only stored corn mill. Each time the windmill was destroyed the animals were determined to prove to Man that they could fight all odds and build the impossible--the windmill.
The animals are eager to prove that they are successful. With the encouragement of Squealer, they work on to demonstrate their ability to persevere and to survive. They believe that the windmill will make life and make work easier, that they will reach their "golden" era by being able to relax and enjoy the fruits of their labor. Their strength in this belief of happiness to come is what propels them forward.