Ambitious characters make my plot move. They look at the world around them, shake their heads and declare "Nah, that won't cut it." Then they do what they can to make it change.
Sometimes they'll be impulsive and hurl themselves headlong into danger, headless for consequences and intent solely on their goals. Sometimes they'll be careful and considerate, and they will plan their next action before they try to achieve anything. It doesn't matter. They have an ambition, and from it is born a story.
It's no surprise that ambitious characters are often villains. Heck, I think today "ambitious" is a negative term for many. And I love my villains (when I have a character that can be easily coined with this term, which is far from frequent), but I think if every ambitious man (or woman) is a villain, then you're limiting yourself.
I think most visionaries are men with ambition. They see the big scheme of things, and they dream of a better world. Revolutions are never easy, but when you have characters that have the strength of character and the means to make them happen, you can easily craft an epic tale.
All it takes is one proactive character acting to get all the reactive characters involved.
But there's more.
This character will carry the plot on his shoulders, and whenever you are at a loss for what to write next, you just have to turn to him, and he will tell you what he wants, and how he thinks he can get it. No matter what, he will strive to achieve his goal, and on his way there, he will create conflict.
When you have an ambitious character, he becomes your plot. You don't need to force it; it'll happen whether you like it or not. And I, for one, like it very much.