Aristotle wants to go on record: pain is a bad thing and should be avoided.
Pain is bad in a general sense. But it can also be bad because it's an impediment to other things.
The opposite of the bad, then, is the good.
Do you see where this is headed? (Hint: pleasure is good).
Moreover, a pleasure might be the best thing. If pleasure is the activity of characteristics (i.e. virtues), the pursuit of pleasure might be happiness—which is the best thing ever.
Happy life = a pleasant life. Happiness is complete in itself, but requires other things to remain unspoiled (health, enough money to feed ourselves, good fortune).
Also, consider that everything, including animals and humans, is looking for pleasure. How, then, is this not the best thing?
We all pursue different pleasures according to our "characteristics," but essentially, we're going after the same thing.
It's a mistake to think that the term "pleasure" means only bodily pleasures, but it's easy to see how this happens. People like bodily pleasures above everything else.
Aristotle ends this chapter with a zinger: if pleasure weren't good, why is it that happy people live pleasantly?