Donald Trump — to feed my ego, which is vast and hungry.
Ted Cruz — because I want to turn the USA into a theocracy. Besides, I'm really mean, and this will give me the opportunity to be mean to lots more people.
Marco Rubio — I wanna get rich, working for Wall Street.
Michael Bloomberg — I wanna get even richer, and make my friends even richer.
Bernie Sanders — Viva la revolucion!
Hillary Clinton —I deserve it! Besides, women's rights!
Bozhe moi! (Which just means OMG in Slavic languages, but it sure sounds impressive.)