How we cite our quotes: (Book.Chapter.Bekker #s); all Bekker line numbers are approximate, since they are keyed to the original Greek.
Quote #1
If, therefore, there is some end of our actions that we wish for on account of itself, the rest being things we wish for on account of this end, and if we do not choose all things on account of something else—for in this way the process will go on indefinitely such that the longing involved is empty and pointless—clearly this would be the good, that is, the best. (1.2.1094a19-23)
Aristotle explains his idea of the most "choiceworthy" thing. He doesn't say specifically what it is at this point (why would you keep reading?), but he does make it clear that we yearn for the good that's most complete. When we stumble onto that certain something that doesn't need anything else to make it better, we should choose that as the greatest possible good in life.
Quote #2
And the person lacking self-restraint acts out of desire, but he does not do so from choice; the self-restrained person, conversely, acts from choice but not out of desire. And whereas desire opposes choice, desire does not oppose desire. Desire has to do with what is pleasant and painful, whereas choice has to do with neither the painful nor the pleasant. (3.2.1111b14-17)
Aristotle begins to narrow the scope of what is truly voluntary by defining the types of actions subject to choice. Desire, it seems, has the upper hand in each of these situations. Because desire comes from a different place than rational thought, it doesn't easily obey our intellect. And without intellect, Aristotle will later tell us, there's no real capacity to deliberate and choose.
Quote #3
What sort of thing is choice, then, since it is none of the things mentioned? It indeed appears to be something voluntary, but not everything voluntary is an object of choice. But is it, therefore, at least an object of prior deliberation? For choice is accompanied by reason and thought. (3.2.1112a13-16)
The answer is yes: choice specifically is the product of prior deliberation. He'll go further to say that we can't properly choose if we are ignorant or without virtue. While this definitely takes away some personal responsibility from those who do stupid things, it doesn't let them off the hook. The inability to choose may keep us from being truly unjust, but it doesn't make us good or honorable, and therefore excludes us from happiness.