Final answer:
Boxer keeps working hard despite his injury because he wants to set an example for the young animals.
Step-by-step explanation:
In chapter 9 of the book, Boxer keeps working hard despite his injury because he wants to set an example for the young animals. Although he may be injured and exhausted, he believes that his dedication and hard work will inspire others to follow his lead. This shows his strong sense of responsibility and commitment to the farm.