I will take this from a socio- economic point of view. If we examine the rivalry between the Socs and the Greasers from a class standpoint, the work reflects how those individuals who are marginalized seek to find some level of belonging from a social order. The Greasers are marginalized, lacking socio- economic power in the class setting. Infact, the Socs are shown to be more socially destructive, but because of their economic status, they are made to belong more:
The rich Socs "jump greasers and wreck houses and throw beer blasts for kicks, and get editorials in the paper for being a public disgrace one day and an asset to society the next." The poor greasers, conversely, "steal things and drive old souped-up cars and hold up gas stations and have a gang fight once in a while.
In this light, one can see how the book makes the argument that class and economic standing play a major role in the notion of exclusion. Those who are excluded, such as the Greasers, have to turn to others, to one another, in order to find some level of belonging for they are not experiencing it in the general social configuration. Thus, the book argues that class and class systems play a large role in determining belonging.
Even the title, "The Outsiders," alludes to the fact that some people are "outside" the lines, the groups, the circles that matter, and, therefore, don't belong. On the flip-side, because they've turned themselves (The Greasers) into the Outsiders, they have created a family and a place where they do, in fact belong. The book has to do with those who don't fit it with others, be it Socs or Greasers, and look to find similarities in a world that is determined to keep them apart, or feeling like they don't belong.
The Greasers are not just a gang, but they're like family to each other. For some of the boys, like Johnny, they are more of a family and a better family than his real family. The Greasers care about each other and watch out for each other. Johnny and Ponyboy are the two that the older boys always look out for, and seem to have a lot of hope for. Darry has a lot of hope that Pony will be able to rise above their lot in life and really accomplish something. Ponyboy's family is very tight-knit, and it is his family that encourages him, protects him, and hopes for a better life for him. At the end of the book, though the "family" of Greasers have lost some of their number, Pony and his brothers know "if we don't have each other, we don't have anything." Belonging to the Greasers gives them all a sense of identity and a family.