Mammon spent the most time in the human world out of all his brothers. Belphie only started to move past his hatred of humans. Lucifer usually was too wrapped up in whatever whim Diavolo had cooked up most recently to indulge. Levi would rather stay cooped up in his room. Beel did not notice much beyond food and his brothers. Asmo bounced around, more interested in sex than commitment, never staying for more than a handful of nights with any one person. Satan rotated between so many events in the Devildom and Human realm that he rarely stayed in one place longer than Asmo. While Mammon spent years working with the same group of witches who regularly called him to the human realm. This meant he understood in painful clarity exactly what six months away meant for his human. How, unless he and his brother’s convinced their human otherwise, each trip back to the human realm would steal more and more from them. Lines would etch themselves onto their face (little at first, but always growing deeper and more prominent), gray would weave itself into their hair, joints would creak, vision blur, hearing fade, and more and more would fall away until all that was left of their human was an empty shell fraying at the seams.
Mammon never wanted to see his human succumb to that. But short of confining them to Devildom for the rest of eternity, an option Mammon cannot afford to think on (he is the Avatar of Greed, it is not in his nature to relinquish his hold over what he treasures), his human’s fate has already been spun, woven, and cut. No spell can truly ward off Death. The best only delay it. Nor can any short of his Father alter the nature of a person's soul. Demons are created through his Father’s Punishment for those who work against His Plan, Angels through His Judgment, and Humans His Love. Even if Mammon got his human to work again his Father, not that he knew how he might manage that, they likely would remain human. For Father is All Seeing, All Knowing, and has a taste for ironic punishment. Giving Lucifer, who wanted to be free to protect Lilith, all the power to rise up as one of the single most powerful demons in all of Devildom only to condemn Lilith to Death. Mammon would be more idiotic than his brothers’ always claimed he was to expect mercy from his Father.
So instead he hoarded. Every smile, every laugh, every confession, every touch, every sharing of space, carefully seared into his memory. Every plan he wants to embark, sight he wants to share, place he wants to visit with them he throws the two of them headfirst into. He does not have time for regrets. He pushes away everyone who tries to intrude. He is his human’s first. Collecting as many moments with his human as he can claw out of every breath in Devildom. It will never be enough, but it’s the best he can do to prepare for the inevitable.