Virtuous actions are voluntary, we want to say, for "the end... is what we wish for, the means what we deliberate about and choose". Yet we like to think that people don't actually choose vice, but only happiness: "no one is voluntarily wicked nor involuntarily happy". Aristotle wants to attack this because it obviously threatens his notion that both virtue and vice are choices.
It could be that a man is ignorant of the means of happiness, but it is notable that legislators do not agree. They say ignorance of the law is no excuse. It may be that a man is careless, and thus his slack life ends up in his being unjust or self-indulgent.
But Aristotle says all this is a choice. You can suppose a man is ill involuntarily, but maybe in the past he had the choice, and disobeyed his doctor. So we could say that he threw away his chance to be well. Thus blindness from birth is one thing, but blindness from drunkenness another.
Of course, we inherit some virtue by nature, but the good man adopts the means of virtue voluntarily. Thus virtue consistently practiced becomes a state of character, as one who trains for a contest becomes excellent by training.
Clearly, Aristotle finishes, all our actions are voluntary: we have a choice each time we act. We start in the right direction, but the progress of our state of character is not so obviously voluntary, just as health is not the result of specific actions. But since we had the power "to act in this way or not in this way" states of character are voluntary.
And that takes us back to the basic position. Virtue, the state of character, is the result of right action in the right way at the right time. And vice is the reverse.