you don't understand! he was sweating because he's the unbelievably handsome, most knowledgeable and extraordinarily kind gatekeeper who refuses to attack anyone before consulting his opponent's general practitioner, who is adored and respected by everyone including young impressionable women (makino), baby killers (sengoku), giants (dorry and broggy), children (luffy), clowns (buggy), senior citizens (gorosei), pirate kings (roger) and birds (mihawk), who inherited roger's wardrobe and weapons to signify his status as the pirate king's true heir (fuck ace), whose favorite hobby is saving people from drowning or being eaten by sea kings, who mastered the art of teleporting just so he could stop wars and bloodshed all over the world, who is the most generous guest bringing the most expensive liquor as a gift, whose diplomacy skills leave reverend doctor otohime in the dust, who can telepathically stop aggressive worshippers of the evil celestial dragons from committing atrocities, who understands the importance of healthy lifestyle choices so well that he wouldn't be caught dead propagating smoking on panel (eww, sanji, you should be ashamed of yourself!), who asked teach to scar him so that bad-looking people wouldn't feel too bad about their looks, who is always wearing rags so that those poorer than him wouldn't feel like losers, who has chosen to be an animal rights pioneer by offering a top position on his crew to monster (a monkey), who makes sure his crew always have something to eat, who swims like a dolphin and who laughs like the sun if it were given a voice - and he was sweating because he couldn't bear the thought of a single innocent person suffering (who's who doesn't count). he saved elbaf, and by proxy, the entire world from an imminent magnetic apocalypse and even the name of his world-saving move - divine departure - signifies his saintliness.
and the sweatdrops sparkle more brightly than the diamonds in the treasure chest in which he was found as a child!