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
[...] and we posit the work of a human being as a certain life, and this is an activity of soul and actions accompanied by reason, the work of a serious man being to do these things well and nobly, and each thing is brought to completion well in accord with the virtue proper to it—if this is so, then the human good becomes an activity of soul in accord with virtue... (1.7.1098a13-15)
In order to figure out which characteristics belong to human happiness, Aristotle wants to decide what kind of work is proper to human beings. Hint: it's to live a life of reason, constantly acting in virtuous ways. But this isn't just the work of humans; it's the work of happy humans, since life in accordance with virtue is the highest good.
Quote #2
And the most frightening thing is death, for it is a limit [or end], and there seems to be nothing else for the dead, nothing either good or bad. (3.6.1115a26-28)
Aristotle sums up the problem of mortality in one sentence: existence, pleasant or otherwise, is so appealing because there's always the next thing to look forward to. Without life (or a sense of afterlife), we can't continue or move on. Aristotle posits that this existential fear is the basis of the virtue of courage.
Quote #3
And the more he possesses complete virtue and the happier he is, the more he will be pained at the prospect of death. For to this sort of person, living is especially worthwhile, and he is deprived of the greatest goods knowingly—and this is a painful thing. (3.9.1117b10-13)
Aristotle's speaking of courage and the paradox relating to it: though it is always pleasant to do what is right, it can sometimes be a painful thing to do. This is especially true in the most extreme case (i.e. sacrificing a good life), since we know that we won't be around to enjoy the greatest pleasure: existing. Why, then, behave virtuously? Aristotle doesn't answer that here. He does imply, however, that courage is a virtue that can be had in degrees—it's okay if we aren't willing to go all the way and die for our principles.