You can tell a person's characteristics by what pleases or pains them, says Aristotle.
If they whine and complain about giving up something they like, they're not moderate in pleasure.
This is because moral virtues deal in pleasure and pain. Pleasure makes us do some crazy, immoral things. Fear of pain keeps us from being noble.
In order to feel pleasure in the right things (and to be pained at the right things), we have to raised well from childhood.
Pain can also be a cure for pleasure (i.e. punishment for "loose" behavior), since we're disciplined by suffering the opposite of the thing we sought.
So we become better (or worse) by learning what kinds of actions produce pleasure or pain.
When we combine these experiences with reason, we should be able to figure out acceptable behaviors.
You might see that virtue and vice have some things in common. In both, we're pursuing pleasure and avoiding pain. A right thinking person just finds pleasure in virtue.
Pleasure is a very strong motivator and very difficult to ignore—but good things happen when we do something that's hard for us.
Aristotle believes that those who can handle the pain/pleasure experience well will be good people.