Grave of the Fireflies

In the final months of World War II, 14-year-old Seita and his sister Setsuko are orphaned when their mother is killed during an air raid in Kobe, Japan. After a falling out with their aunt, they move into an abandoned bomb shelter. With no surviving relatives and their emergency rations depleted, Seita and Setsuko struggle to survive.
Why do fireflies have to die so soon?