Friend needs help with Plato's Republic

[Sonic]

here is his post to me. Any help or ideas are appreciated.
full text of the republic here: http://www.fordham.edu/halsall/ancient/p...public.txt

Colin L.:
I have hit a wall and really need some help. I am writing a paper on the Republic.
I need to describe how (619b-d) [the last 3 pages of the Republic mainly focusing on the myth of Er and rebirth cycle] sheds light on the overall argument of The Republic.
Describe how the following event sheds light on the overall argument of The Republic:

“the prophet said at the time: "Even for
the last comer, if he chooses wisely and will live diligently, there
is appointed a happy and not undesirable existence. Let not
him who chooses first be careless, and let not the last despair."
And when he had spoken, he who had the first choice came for-
ward and in a moment chose the greatest tyranny; his mind
having been darkened by folly and sensuality, he had not
thought out the whole matter before he chose, and did not at
first sight perceive that he was fated, among other evils, to de-
vour his own children. But when he had time to reflect, and
saw what was in the lot, he began to beat his breast and lament
over his choice, forgetting the proclamation of the prophet;
for, instead of throwing the blame of his misfortune on himself,
he accused chance and the gods, and everything rather than
himself. Now he was one of those who came from heaven, and
in a former life had dwelt in a well-ordered State, but his virtue
was a matter of habit only, and he had no philosophy. And
it was true of others who were similarly overtaken, that the
greater number of them came from heaven and therefore they
had never been schooled by trial, whereas the pilgrims WhO
came from earth, having themselves suffered and seen others
suffer, were not in a hurry to choose. And owing to this inex-
perience of theirs, and also because the lot was a chance, many
of the souls exchanged a good destiny for an evil or an evil for
a good. (619b-d)” -Republic

So, my problem is that I cannot prove/show that justice is good in and of itself because I view it as a means to another gain/reward. (Mainly coherence & protection between and from people, and as Plato describes in the Myth: the just soul will make the right decisions and as a result will be happy/healthy.)
One could say that Justice is good because it creates a healthy soul which in turn makes good choices. But that is in direct conflict with Plato’s stated goal: to show that justice is worthwhile even in the absence of the rewards it might confer.
I am in a loop here, and any help would be welcomed.
Thanks