Aristotle on virtue, fortune and happiness

Aristotle, in Book 1 of his Nicomachean Ethics, introduces the following puzzle: "Is happiness acquired by learning, or habituation, or by some sort of other form of cultivation?  Or is it the result of some divine fate, or even of fortune?" (Aristotle, 12)  This is actually quite a puzzle, since Aristotle claims that we need certain "external goods" to be added in order to attain happiness.  Addressing this argument, how does Aristotle handle this aspect of luck or fortune with his treatment of virtue in the larger argument?  Is the attainment of happiness, by your assessment, just a matter of luck?