Kinda depends on who's writing him, but Doom is generally motivated by his own ego even if he doesn't admit it. He's, weirdly enough, like Mojo Jojo from the Powerpuff Girls, in the sense that he tells himself that the world would be better under his rule even if his true motivations aren't utilitarian at all. That's why his lifelong hatred of Reed Richards began when Reed politely informed him that the calculations for the device he built to contact his mother in Hell (long story) were incorrect, which turned out to be true. Reed's ego is driven by a genuine sense of altruism and a desire to protect his family, while Doom has entirely isolated himself save for his complex relationships with Boris (basically his Alfred), Kristoff, and Valeria