How we cite our quotes: (Book.Chapter.Bekker #s); all Bekker line numbers are approximate, since they are keyed to the original Greek.
Quote #4
Since what is chosen is a certain longing, marked by deliberation, for something that is up to us, choice would in fact be a deliberative longing for things that are up to us. For in deciding something on the basis of having deliberated about it, we long for it in accord with our deliberation (3.3.1113a10-13)
This is choice in a nutshell. Note that in order to choose properly, three things have to be present: 1) longing (preferably correct longing); 2) deliberation, or the ability to think before acting; 3) the ability to act under our own steam, not out of compulsion. If any of these are missing or incomplete, we won't act out of choice but out of compulsion, desire, or necessity.
Quote #5
In this way too, it was possible at the beginning for both the unjust person and the licentious one not to become such as they are, and hence they are what they are voluntarily; but once they become such, it is no longer possible for them to be otherwise. (3.5.1114a19-22)
Choice plays a huge role in the development of our virtues. When we're in the position to deliberate and choose our actions (i.e. what we do is voluntary) we're choosing also the type of person we are becoming. If we choose poorly, we're habituating ourselves to become bad people. And once we walk down that path, there's no turning back because eventually, our reason (and hence, our ability to choose properly) will be destroyed.
Quote #6
(It is not in making having the capacity to boast but in making the choice to do so that someone is a boaster, for choice accords with one's characteristic, and he is a boaster because he is that sort of person). (4.7.1127b14-16)
It's important to remember that characteristics or virtues can't shape our personality unless they're active in us (i.e. they are translated into action). We can't be called a good person until we actually do things that are good. It also works the same way for the annoying stuff. While we all have the capacity to boast, it's the choice to do so that motivates us to action—and therefore gets us pegged as an obnoxious exaggerator.