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
Neither by nature, therefore, nor contrary to nature are the virtues present; they are instead present in us who are of such a nature as to receive them, and who are completed through habit. (2.1.1103a23-25)
Aristotle engages a bit in the "nature vs. nurture" argument in Ethics, though he avoids the topic of natural goodness (or badness). He clearly believes that we are born disposed to receive virtues, but that our education and habituation—practical application of principles to daily action—have the upper hand in forming our characters. This also speaks to the idea of personal responsibility. Though education (i.e. what our parents and communities instill in us) is crucial to forming a moral human being, it's ultimately up to each person to stick to what he knows is right.
Quote #5
So too in the case of the virtues: by doing things in our interactions with human beings, some of us become just, others unjust; and by doing things in terrifying circumstances and by being habituated to feel fear or confidence, some of us become courageous, others cowards. (2.1.1103b14-18)
Becoming a person of virtue requires a constant practice of moral principles (or "habituation"). We learn what is expected of us through education (how to be courageous, or just), but unless we have the characteristic to receive such learning, it'll be hard to put it into action. But it is by acting as a just or courageous person that we actually earn the titles.
Quote #6
For moral virtue is concerned with pleasures and pains: it is on account of the pleasure involved that we do base things, and it is on account of pain that we abstain from noble ones. (2.3.1104b9-11)
Though Aristotle will later tell us that pleasure might very well be the highest good for humans, it's also a bit of a trap. Since we're motivated from the beginning to seek happiness, we are naturally drawn to that which is pleasant. But depending on the virtues present (or absent) in each person, pleasure can lead us away from doing what is right.
On the one hand, we might abandon everything else to seek pleasure. On the other, we might love pleasure so much that we become reluctant to endure pain. And as we will see, pain is sometimes a necessary part of virtue.