A great quote on ambition from John Stott, via the blog That Happy Certainty:
Ambitions for self may be quite modest. . . . Ambitions for God, however, if they are to be worthy, can never be modest. There is something inherently inappropriate about cherishing small ambitions for God. How can we ever be content that he should acquire just a little more honour in the world? No. Once we are clear that God is King, then we long to see him crowned with glory and honour, and accorded his true place, which is the supreme place. We become ambitious for the spread of his kingdom and righteousness everywhere. (The Message of the Sermon on the Mount (IVP, 1993), 172–173).