Ambition is a dogged persistence towards a goal to accomplish something. We are ambitious for our families, to make a better life for them. We are ambitious for noble causes of all kinds. We should be ambitious for God, to see his Kingdom come on earth as it is in heaven. Ambition is essential.
Ambition is a good thing.
But second, ambition is too easily entwined with other motivations. We wrap ambitions in our own sense of self-worth, and so achievement of our goals is no longer pure. It is no longer to accomplish lofty goals. It is also to vindicate our own selves, and to highlight our own achievements. And so it becomes selfish, self-serving.