Human virtue is the central topic of Aristotle Ethics, and it is worth considering why this should be so. Aristotle's supreme good is the well functioning of the human being qua human; functioning well is nothing other than 'activity in accordance with virtue (or excellence)'; and this he interprets so that practical virtue (as ordinarily understood) becomes the focus of attention. So of course Aristotle Ethics is concerned mainly with virtue and the virtues and would not be what we know as Aristotle's Ethics otherwise.
But to illuminate his preoccupation, let us not take for granted, as if it were a fact of nature, that Aristotle's is a 'virtue-oriented' ethics. Why this perspective? It is not enough to say that he follows Plato, since in many things Aristotle goes his own way. The topic at hand provides an example. Both philosophers hold personal excellence to be of the essence of human well-being and both regard it as a state of the soul; but these similarities come with a striking difference. Aristotle constantly reminds his readers that happiness is activity: it is virtue in action, not virtue unused. And virtue as a state of the soul is of value only for the activity which it makes possible (cf. 1098 b 30-1099 a 7). Plato, by contrast, writes in the Republic as if that harmonious internal state which he calls 'justice' were something good and beautiful in itself apart from the external actions through which it is expressed. Thus he is able to reach a position which Aristotle decisively rejects, namely that virtue, as distinct from virtuous activity, is the supreme good for man.
The Aristotelian emphasis on activity is well to the fore in that early passage where the term 'virtue' first appears as an element in the definition of happiness (1098 a 7-18). Leaning on the flute-playing example, Aristotle rewrites 'acts (or functions) well' as: 'acts (functions) in accordance with excellence (or virtue)'. Is the rewriting justified? What is the point of it, justified or not? We must take up these questions now, since from here on in the text the Aristotelian connection between happiness and virtue will be doctrinally established.