Whether or not there's One Good Thing from which all other good things come, Aristotle says that, in each science, the good thing is the end for which it aims.
And sometimes that's more than one thing.
And sometimes, we aim at those things for the sake of something else (i.e. wealth).
Those kinds of goods are incomplete.
The "better" good, then, is something that is complete—something that's good in itself, without the need to refer to something else.
And if there are several complete goods, then the most complete is the best one.
So, to recap, the complete thing is the one that's chosen for its own sake—not as a means to get at another good.
Aristotle concludes that happiness is exactly this kind of good. We always choose happiness for its own sake.
We choose other virtues (i.e. honor, pleasure, intellect) as ends in themselves, but also because they help us to be happy.
On the flip side, nobody chooses happiness to become virtuous. Therefore, it's a complete end in itself.
Aristotle introduces the concept of self-sufficiency. A complete good is self-sufficient—and not just for one person by himself, but for those around him.
It is the thing that makes life "choiceworthy"—something we can consciously choose regardless of any other good—what Aristotle believes happiness to be.
Further, happiness is the most choiceworthy thing, since it isn't just one good thing: it's lots of good things taken together. If you added more good things to it, you'd have a "superabundance" of good things.
But what is happiness?
We might agree that it's the best of all good things…but how to define it?
To do this, we have to first define the work of humans. Since good can only be determined for specific sciences (i.e. medicine, warfare), we have to define the category to find the relevant good.
It can't just be living—life associated with growth—since even plants do that. And it can't be sense perception, since all animals have that.
Aristotle settles on this definition: an active life associated with reason.
This includes not just having reason, but also applying it through thought and action.
Further, he defines human work as "activity of the soul in accordance with reason"—and the work of a "serious man" (one dedicated to moral virtue) is to work well and nobly.
So, human good has something to do with activity of the soul in accord with virtue.
But not just any virtue: the most complete virtue. Remember, that which is most complete and self-sufficient is the absolute best, in any category.
Aristotle makes one more proviso: all of this must be wrapped up in a "complete life"—not one that's too young or immature.
So now we have a rough sketch, but a good one, concerning happiness and the purpose of human life.
Aristotle tells us that such an outline can be filled out later, if only it's good.
He asks us to remember what he said earlier, that sometimes it's best not to be too precise about things or seek their origin. To do so may distract from the purpose of the inquiry.
Principles can be sought, but each according to its own nature: some through observation, others through reasoning and "habituation."
And we should seek them, since knowing the beginning "seems to be more than half of the whole," revealing useful things once it's known.