People are different. They have different political stances, personalities, religious beliefs, views about money, and so on. Despite those (and other) differences, people are far more alike than they are different. This is why Cherry is probably the most important character in the entire novel. She isn't necessarily featured on many pages overall, but her impact on the novel and its characters can't be overstated. She is the character that points out to Ponyboy that the Greasers and the Socs are simply two sides of the same coin. Both groups have their problems and issues. Both groups seek out violence as a way to deal with those problems. Both groups have an "us vs. them" attitude. It's this attitude that makes the relationship tense, but it also negatively impacts the wider society as a whole.
A society is a singular unit. The world might have societies all over the place, but a society is singular. Consequently, it can be thought of as a singular unit in the same way that a team can be thought of. Individual players have different talents and skill sets. Individual players might have different beliefs and value systems, too; however, a team works as a cohesive unit for the betterment of the team. The fact that the Greasers and Socs are looking out for themselves means that they are not looking out for the "team" that is larger society. They are perpetuating division rather than unity and tolerance, and that negatively impacts the society that they are living in.