According to Aristotle in the Nicomachean Ethics, what is the chief good at which all human activity and desire is directed?
I think that the best answer to this is that all human beings are trying to find happiness.
Aristotle says that everything we do is in some way aimed at a goal (or "end") that we think is good. Different people can have different ends that they think of as good, but they all lead to one thing. Let's say you and I are different -- I want to make enough money to retire when I am 45 and spend my life writing a novel (I don't). You want to make only enough money to support yourself as you do good works and help the poor. These seem to be different ends. So what do they have in common?
To Aristotle, we are both seeking happiness. These things I have mentioned are just intermediate goals. They are not really what we seek. What we are both seeking is happiness. Once we are truly happy, we no longer have to try to achieve anything more.