For an INFP to be ambitious, they need to feel that they are fighting for something, something that means the world to them whereas SJ/NJ types are less picky, more naturally into structure and duty and work ethic.
I have a very bad work ethic, not because I am lazy, but because doing what I have no concern for sucks my energy so much that as a cashier, I constantly wanted to spend a week just sleeping.
However, if an INFP have found her/his 'mission', they can be more dedicated than any of the other types, to reach their ideals because we have such strong visions.
We are not naturally bulldozers, but that doesn't mean we can't 'learn' to be or 'force ourselves to be'. It would be extremely energy draining, though. This is why sometimes I wish I am an extrovert, for energy purposes. I sleep more than anyone I know. My friends all think I have some kind of iron deficiency.