How does James Jarvis react when Kumalo tries to thank him for all the good work he is doing in Ndotsheni? Why does he likely react this way?

Kumalo tries to tell Jarvis that he is the best white man he has ever met, but Jarvis chafes at the compliments. He does not think he has done anything particularly spectacular, especially compared to the sacrifice his son made. Jarvis further likely is sad that it took the death of his son to jar him out of his own complacency and into taking action to help others.