We should aim to do what is good. Good is an end, the purpose of an action. Happiness is a purpose of an action.
1. The only proof capable of being given that an object is visible is that people actually see it. Similarly, the sole evidence that something is desirable is that people do actually desire it.
2. No reason can be given why the general happiness is desirable, except that each person desires their own happiness.
3. Each person takes their own happiness to be good, and so, adding each person's happiness to that of others, the happiness of everyone, the general happiness, is good for people in general.