I cried when Hazel went with El-Ahrairah at the end of the book when I first read it - it's beautifully done. The book was adapted very well done on Radio 4 recently, too, and worth looking out for. I never really liked any of his other books, but I think this one deserves to join the classics.
It was beautifully done in the film, as well.
Richard Adams was 96, and it seems, like Hazel, he was feeling tired, so he left his body behind to go with the Prince with a Thousand Enemies and join his Owsla.