You could link this theme in with the idea of class consciousness and how the world of this novel presents us with two very stratified groups that seem to shape so much of your life whether you want them to or not. Being born into the working class means that you are a "greaser" and because of that you are looked down on by society. As Ponyboy says in his introduction, Socs are attacked by the press one minute and then praised for their contribution to society the next. Greasers receive no such ambivalence from society. They are the outcasts, the "outsiders" whether they like it or not.
Apart from this general treatment of the unfairness of life, for a specific example, you might want to look at Darry. In Chapter Eight, Two-Bit makes a fascinating remark, saying that the only thing that keeps Darry from being a Soc is the gang. Ponyboy comments:
I had known it for a long time. In spite of not having much money, the only reason Darry couldn't be a Soc was us. The gang. Me and Soda. Darry was too smart to be a greaser. I don't know how I knew, I just did. And I was kind of sorry.
Darry had the necessary talent to go to college and receive a scholarship, however the unfortunate demise of his parents meant he had to become a breadwinner and get a job instead. Life was certainly not fair for Darry, forcing him into a menial job when he was capable of making so much more of himself. This is, of course, why he is so hard on Ponyboy--because he wants him to have the opportunities that life did not give him.