Malbim: The first losses were via nature - losses that come via the elements of wind and fire, and via people who damage. Hashem only removed His Hashgachah, and the Satan was able to bring it. Now Iyov will be afflicted via boils and illnesses. Based on nature, these afflict one who ate too much. A Tzadik guards himself from excess food and intimacy - "Tzadik Ochel l'Sova Nafsho." The Satan needed Hashem to use His Hashgachah against Iyov's nature.