Mammon is a selfish being, even for a devil. He is a patron of material greed and is frequent in his betrayals. His treacherous nature has made him very unpopular with the other Lords, meaning none will even consider striking an alliance with him. With no opportunities for alliances left in the Nine Hells, he has instead he turned to the Prime Material Plane to gain power, corrupting rich and powerful mortals to become greedy and lecherous monsters and eventually signing their souls over to him, essentially banking on the old saying "the love of money is the root of all evil." As a result of having no friends, the Lord of the Third is a very paranoid devil and will punish his servants at the slightest provocation by demoting them. This high turnover rate means that Mammon also promotes more devils more frequently than any other Lord, and will pay a good price for a soul fitting for a specific promotion. Noticing the cycle here?
He once had an affair with Glasya, but after his role in the Reckoning Asmodeus forced his daughter to end the relationship, and left Mammon with a new form consisting of a male humanoid upper torso and a snake-like lower body as a mark of shame. Both Mammon and Glasya seem to bow to Asmodeus' command, but some claim that now that she's no longer right under her father's nose Glasya has rekindled the relationship. Others claim that she despises Mammon for not fighting for their relationship. It's even possible that both are true, though how they manage an affair around Old Scratch turning Mammon into a snake-man to give him the "Walter Peck" treatment is a riddle for the ages. Whatever the case, since Glasya's ascension Mammon has redoubled his effort to increase the flow of souls to his realm by corrupting the Material World via appealing to its inhabitants' greed, and increasing his power when they sign over their souls to him or his minions. Unlike Baalzebul's punishment, Mammon can re-adopt his original form of a bloated Pit Fiend when he wants, but only for 10 minutes per Hit Dice a day, whereupon he must endure the frustration of turning back into his new form, which he takes out on his servants and his endless quest for more riches and souls.
It's worth noting that he lives in a city perpetually sinking into a swamp, so one can't help but wonder if this might be connected to the fact he sits on a pile of more gold than even a dragon would ever think they needed.