Sometimes, Sanji looks at Zoro and wonders how the man can possibly live without a woman in his life, whether or not she returns his affections. To Sanji, not having a beautiful lady to lavish attention and dance attendance on is not actually living – it's simply existing.
How can Zoro really live without a woman?
But when he hears Luffy's happy laughter when the captain is with Zoro, when he actually sees Zoro smile at Luffy's antics, when he is woken at night by the creak of the floorboards as Luffy slides out of his hammock to snuggle in with Zoro on the small couch, for warmth or comfort from a nightmare or simply for companionship, he thinks he understands.