Aristotle begins with "perplexities" about self-restraint. For instance, can a learned person lack self-restraint? To Aristotle, this seems impossible.
Socrates didn't even believe in lack of self-restraint as a thing, since he felt that no one would reasonably act against his own best interests.
But Aristotle says that we do this kind of thing all the time—and we need an explanation for this.
Some say that a person who gives in to wicked desires isn't a person of strong conviction. They have morals, but can't stick to them. Aristotle thinks these guys should be pitied.
But really corrupt people can't be pitied for lack of self-restraint.
More perplexities follow about how we resist wicked desires. Does prudence help us? Surely prudence can't come within a mile of a person without self-restraint...
Also, there's this: only a person who has wicked desires in the first place can actually have self-restraint (otherwise, he wouldn't need it).
Should we praise such a person when they resist as they should? It seems that moderation deserves the kudos, since the moderate person doesn't even have base desires in the first place.
Self-restraint can also be bad if it leads us to conform to opinion unquestioningly—especially if the opinion is a bad one.
This might be something that's culturally acceptable, but not morally.
On the other side, a lack of self-restraint might allow a person to disregard bad opinion and make better choices. (Think of whistleblowers and rebels who advocate for social change.)
Aristotle continues to add other perplexities concerning self-restraint from other philosophies.
A person who behaves badly at the suggestion of others is less bad than one who lacks self-restraint. After all, he can be persuaded to behave himself better, too.
One last question: can we lack self-restraint generally, or only toward specific things?