[...] 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.
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.
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.
And pain unhinges a person and destroys the nature of him who undergoes it, whereas pleasure does no such thing. (3.12.1119a24-25)
This simple observation on the nature of pain reveals something larger about human existence. Though living is, in itself, a great good, our ability to be virtuous and therefore happy is dependent on several factors. We can see here that when it comes to pain, all bets are off. It can erase our virtues and our intellect, and it can take away our power to make good choices. It's because of this transformative power that Aristotle says cowardice is more forgivable than other things, since it's our fear of pain that motivates it.
Metaphorically and in reference to a certain similarity, there is something just that pertains, not to a person in relation to himself, but to certain parts of himself...In these sorts of arguments, the part of the soul possessing reason is set apart from the nonrational; hence to those who look to these considerations, there does in fact seem to be injustice in relation to oneself, because in these parts of the soul, it is possible to suffer something contrary to their respective longings. (5.11.1138b6-13)
Aristotle's struggled with this question throughout Ethics up to this point: can we be unjust to ourselves?
His first answer says no: justice only exists for those who live in community, and according to laws.
But he wants to finesse this answer a bit more—perhaps to account for self-harming behaviors. In doing so, he gets very close to a psychology or inner life of a human being: we can be internally torn and we can rebel against our own better impulses and longings. In this way, we're a community unto ourselves, and reason is meant to be the ruler.
So let those things by which the soul attains the truth, by way of affirmation and denial, be five in number. These are art, science, prudence, wisdom, and intellect (for through conviction and opinion, one can be mistaken). (6.3.1139b15-18)
Aristotle tells us that it's through these five capacities that we perceive the world around us and make sense out of our observations. These are the intellectual virtues, and when guided by correct reason and the moral virtues, humans have a pretty good chance of getting at what's both good and pleasant in life.
And every art is concerned with the process of coming-into-being, that is, with artfully contriving and contemplating how something that admits either of existing or not existing may come into being, the origin of which lies in the person making but not in the thing made. (6.4.1140a11-14)
When Aristotle speaks of art, he's not exclusively speaking of the kind of thing you might find in an art gallery. Art consists of a technical knowledge that produces something—shoes, houses, furniture, clothing—and coincides with our modern understanding of craft (and craftsmen). Art deals with "things that admit to being otherwise"—those things that aren't part of fixed knowledge and culminates in the creation of something, rather than simply in action. It is a way of getting at truth, and it represents how we interact with our world and interpret it.
They define living in the case of animals as a capacity for perception, and in the case of human beings as a capacity for perception or thought. But a capacity is traced back to its activity, and what is authoritative resides in the activity. So it seems that living is, in the authoritative sense, perceiving or thinking. And living is among the things in themselves good and pleasant... (9.9.1170a16-20)
While it seems that Aristotle is equating human perception with animal perception, that's not the case here. Human perception is marked by intellect, comprehension, and deliberation—an active engagement with the world around us. Though this is little more than contemplation (i.e. it isn't an activity in the same way that running or mountain climbing is), it is the basic thing that defines us as human and the thing that is most pleasant for us to do. Coincidence?
The result is that if we are perceiving something, we also perceive that we are perceiving; and if we are thinking, that we are thinking. And to perceive that we are perceiving or thinking is to perceive that we exist...Moreover, perceiving that one lives belongs among the things pleasant in themselves, for life is by nature a good thing, and to perceive the good present in oneself is pleasant...(9.9.1170a32-1170b2)
Try not to think about this one too hard. If you're familiar with René Descartes "I think, therefore I am," you've grasped the phrasing. Aristotle points to the awareness of our own existences as key to the human experience, and the greatest pleasure that we have as living creatures.
This is because life's already a good thing—without it, there's nothing—but to the good person, existence is even better. Such a person has much to love about himself and also what he needs to be self-sufficient. To contemplate so much good, then, must be the very best pleasure of life.
Pleasure also completes the activities, as indeed it does in being alive, which people long for. It is reasonable, then, that they aim also at pleasure, since it completes for each what it is to be alive, which is a choiceworthy thing. (10.4.1175a16-17)
Aristotle's trying to convince us that pleasure may be the premier good thing about being alive. And he's doing a pretty good job of it. He says that life itself is a kind of activity within which we choose other activities that accord with our virtues. While each of these activities are good in themselves, they are made even better by pleasure.
Pleasure, then, completes human activity, which means that a clever human being might, in fact, be doing the things he does specifically because pleasure is the end he's aiming for from the beginning.