Modern stories, especially in the speculative fiction of the local book store avoid ambition like the plaque
OK, let me rephrase that. In modern stories ambition is seen as a flaw rather than a boon. For example, many a changeling story (you know the ones were the ordinary kid turns out to be a prince, champion or wizard) has ambition at the core of the heroes motivation, mostly the ambition to escape their current circumstances into a better (and more fascinating) world.
After all ambition, by definition, is not necessarily bad. In fact we celebrate it all the time, in celebrities and businessmen, that is until they succumb to it and then it becomes a moral about how power and greed corrupts the human soul. Since ambition is usually tied with power it has become synonymous with greed even though it is technically not so. So much so that as in the examples above, ambition must be disguised in someway, most commonly as the opposite, that is, the hero doesn’t really want the power but has no choice but to take it, even though he tends to enjoy at least some of the perks that come with it (like Quidditch!).
If ambition, by itself, is not inherently evil, how can a hero be motivated by it in a good way? It depends on what the hero wants, how he goes about obtaining it and what does he do with it once he has it. Mostly the “it” is power, that is the ability to do something. It could be political, wealth or status. The prince might fight to gain the throne out of a sense of duty not only to his murdered father but also to his people, who need a leader. A wizard may quest for a powerful artifact to stop an ancient evil or an industrialist may search for wealth so that he can change the system from within.
Their is always a risk that the hero may be corrupted as ambition morphs into greed or arrogance. The hero is dancing at the edge of the moral abyss risking it all. He may find that what he search for is ultimately futile or useless (but not in a deux machina sort of way, that is you find the ultimate artifact of power and it loses said power the moment he uses it against the ultimate evil) or that he, at some point, might have to walk away from power for the greater good or learn to use said power wisely.
If used right ambition can be a powerful and deep motivation for the hero.