Generic "ambition" is a serious case of putting the cart before the horse. If you have ambition to do something, that's great; if not, deciding you should be "ambitious" and then trying to figure out what to be ambitious about rarely ends well.

I think this is why I get creeped out by ambitious people sometimes. I'd much rather my doctor be passionate about medicine than be someone who decided medicine was more "prestigious" than nursing. As a personal anecdote, I am currently in medicine because I want to specialize in psychiatry. I am passionate about psychiatry and plan to be an awesome psychiatrist. I am not quite as passionate about organic medicine with its heart attacks and kidney infections, and although I work hard at it and am pretty good, some of my classmates who get up every morning super excited because they've dreamed of treating kidney infections their whole lives are better. I don't begrudge them this and if I ever got a kidney infection I'm going straight to them and not to the doctor who went into medicine as a subgoal of something else; if they ever get depression I hope they'll come to me for the same reason.

I understand it's the same in many other fields. Paul Graham writes that successful startup founders start with a problem they want to solve, eg Larry Page and Sergey Brin were frustrated with terrible online search; unsuccessful startup founders decide they would really like to earn fantastic amounts of money and only worry about what business they'll do it in as an afterthought.

The only problem here is charity: I do think it may be morally important to be ambitious in helping others, which might even include taking a lucrative career in order to give money to charity. This is especially true if the Singularity memeplex is right and we're living in a desperate time that calls for a desperate effort. See for example Giving What You Can's powerpoint on ethical careers. At some point you need to balance how much good you want to do, with how likely you are to succeed in a career, with how miserable you want to make yourself - and at the very least rationality can help clarify that decision.