"Since then, as philosophers prove, appearance tyrannizes over truth and is lord of happiness, to appearance I must devote myself. I will describe around me a picture and shadow of virtue to be the vestibule and exterior of my house; behind I will trail the subtle and crafty fox.... But I hear some one exclaiming that the concealment of wickedness is often difficult; to which I answer, 'Nothing great is easy.'"
- Glaucon, The Republic, Book II, by Plato. Translation by Benjamin Jowett
Why praise justice over injustice? Glaucon (and Adeimantus too, but his toga's not nearly as shiny) argues that it is far better to appear just while secretly being unjust. Glaucon compares the perfectly just sistafriend to the perfectly unjust sistafriend. He says the worst kind of injustice is to appear just when you are not, when you manage to convince everyone that you are a great person despite your interests in robbing little old ladies and knowingly spreading syphilis, possibly at the same time.
Everyone loves the perfectly unjust sistafriend. Glaucon says the perfectly just sistafriend shouldn't practice justice for the praise and rewards it brings, or else it could be said that perfectly just sistafriend is totally doing it for the nookie. Glaucon makes sure that everybody hates the perfectly just sistafriend. Bitch is always acting all high and mighty. Well, not really, being perfectly just and all, but what? You think you so much better than the rest us? What now, perfectly just sistafriend?
The question posed by Glaucon is this: Why do all good sistafriends rave about justice when being unjust is way easier and generally involves sexier men with equally loose morals? What's so great about justice in itself that is worth pursuing? Socrates compares the question to reading small print, which is somehow easier to read if you read the same words in a larger size. (I guess... Athens had never seen the fine print on a credit card statement.) Justice is a virtue of both states and individuals, so by examining justice on a bigger scale, it will easier to determine the properties of justice.
Now the kids pull out their legos and build a perfect society in order to figure it all out.