Originally Posted by
Bastian
I can't believe no one has yet posted the correct answer.
At the end of Wind Waker we're treated to a bit of Ganondorf's reasoning. He grew up and lived in the Gerudo Desert, which was very harsh and unforgiving, and longed for a green land like Hyrule. Being the self-styled king of the Gerudo, he decided to invade and take that land for himself and his people.
So I suppose the answer is greed. Why did England constantly claim France and Ireland as their own? And the Americas? And so on.