[I]s there... a kind of good that we choose to have not because we desire its consequences, but because we delight in it for its own sake...? (357b)
Glaucon shows himself to be one of the most perceptive of Socrates's interlocutors. He wants a definition of the good that shows why it's appealing, not just why it's "right." This is a general theme in the Republic, in which characters often ask for things to be defended on the basis of their ability to bring about happiness, not just on the basis of morality.
"Then the good is not the cause of everything; rather it is the cause of the things that are in a good way, while it is not responsible for the bad things." (379b)
Socrates wants to be very clear when he separates the good from the bad. To this end, he makes sure to explain that the good can't produce anything bad; it can only produce good things. This may sound obvious, but think about situations in which good causes are associated with bad results, or in which bad intentions lead to (seemingly?) good consequences for the person acting badly.
"Then the man who makes the finest mixture of gymnastic with music and brings them to his soul in the most proper measure is the one of whom we would most correctly say that he is the most perfectly musical and well-harmonized..." (412a)
Socrates often appeals to the language of music and harmony to communicate moral judgments. Here, he imagines that the moral person is the one best able to find a perfect balance between athletics and music—generally not the most obvious combination.
"...it looks to me as though there is one form for virtue and an unlimited number for vice..." (445c)
This doesn't sound like the best news, does it? Socrates imagines that while there's only one way to be good, there are lots of ways to be bad. Yikes.
"...there is a need for the best men to have intercourse as often as possible with the best women, and the reverse for the most ordinary men with the most ordinary women..." (459d)
Thinking about morality in a more unconventional way, Socrates believes in "breeding" humans so that the best people have children together in order to produce the best kinds of children, and so on. Does that sound like a promising strategy?
"Is there any way in which the orderly man, who isn't a lover of money, or illiberal, or a boaster, or a coward, could become a hard-bargainer, or unjust?" (486b)
The "orderly man" whom Socrates equates with the wise man is demonstrated to be incapable of acting immorally. This seems to prove that that morality can be defended rationally. Are you convinced?
"At all events, this is the way the phenomena look to me: in the knowable the last thing to be seen, and that with considerable effort, is the idea of the good; but once seen, it must be concluded that this is in fact the cause of all that is right and fair in everything." (517b-c)
It sounds more complicated than it is, but all Socrates is saying is that understanding true goodness is the hardest—but also the most rewarding—goal of our lives. It's not just a good end in and of itself; it also opens up the door to understanding other positive things. Sold yet?
"Unless a man is able to separate out the idea of the good from all other things and distinguish it in the argument, and going through every test, as it were in battle... he comes through all this with the argument still on its feet..." (533b-c)
By using this image of a dangerous battle to describe the intellectual work of understanding the good, Socrates makes intellectual labor sound heroic and exciting. Forget the Trojan War—philosophy is where all the action is. It makes sense, really, when you consider Socrates's argument that philosophy is the only "real" thing underlying everything else.
"Surely, when wealth and the wealthy are honored in a city, virtue and the good men are less honorable." (551a)
For Socrates, the desire for wealth and the desire for morality are pretty much incompatible. Does that seem true to you? We can't think of any examples (ahem) from our contemporary world that would back this up... Yeah, not a single one...
"For the contest is great, my dear Glaucon... greater than it seems—this contest that concerns becoming good or bad—so we mustn't be tempted by honor or money, or any ruling office, or, for that matter, poetry, into thinking that it's worthwhile to neglect justice and the rest of virtue." (608b)
Socrates dramatically describes the challenges and dangers he and the guys face on the way, almost as if they're explorers embarking on a great adventure. Temptation is everywhere, but they need to stick to their belief that justice and virtue are the only way to go.