Recall Mill's arguments that everything else we desire for its own sake we desire as part of happiness. There is a disagreement about what sort of human life is most flourishing. Some say it is a life of pleasure, others of honor, others a wealthy life, others a virtuous life. Aristotle will defend some version of the last view. The best human life is one of excellent human activity.
Excellent activity is inherently pleasurable to the virtuous person.
As naturally functioning beings, human beings must have a function. Their function must be whatever is distinctive about them. This is the function of human beings. In Book II, Aristotle turns to the question of what virtue and virtuous activity is. Aristotle aims to argue is best.