Children were often fed that idea that a spanking was done out of love. That a parent didn't do it if they didn't love you. It was a lesson even if at the time it just burned like hell against your ass. That was how it was for Greed. While Sins cared in a twisted fucked up way, Lust was spanking him for his own good. He didn't like it, but when he crumbled in front of her she couldn't help but coddle him after. Just as he couldn't help but cry, just like a child who didn't understand why what they did was wrong.