Why are civilizations divided into social classes?

The answer to this question lies in human behavior and the fact that in most societies, there is a limitation of resources.  I am going to use the term "society," rather than "civilization" because I think "civilization" is too broad a term to address this question.

For better or worse, it seems to be part of human nature to want to feel superior to others. This is what motivates competition. It's wonderful if a person's self-esteem rests solely on his or her own accomplishments, but there is a side to us that likes to look down on others to feel good about ourselves. We cannot seem to help making comparisons. Of course, this drive to be better than others is also what clears land, builds buildings, and got people to the moon, so it might be fair to say we might not have civilization—as we understand it—at all if it weren't for this human inclination. That this is a trait that is evolutionary in nature makes a great deal of sense. You can see, though, that this need makes for winners and losers, so losers get relegated in one way or another to another "class."

Another aspect of human nature that is relevant here, I think, is our tendency to demonize the "other." This was a useful trait early in human history that allowed us to be wary of other tribes who might not mean well. Today, its use is far more malignant. Slaves imported from Africa were "the other" in a way African Americans have yet to recover from in American society. Each wave of immigrants to our shores creates a new underclass, as "No Irish Need Apply" was a common sign in New York City when boatloads of Irish fled famine. More recently, some Americans have been working hard on creating an underclass of Latinos, particularly Mexicans, and I would guess Muslim immigrants will be the latest iteration of this. 

If resources are finite, this also promotes some sort of class system. Without enough land, those who own the land there is will be sure to maintain their stronghold, resulting in a feudal or tenant class below. If water is a finite source, whoever controls water has the power to provide or not provide it, creating an underclass. Those who own diamond mines have become a powerful upper class, with people who are virtually slaves working the mines. The royal family of Saudi Arabia is at the top of the heap because it controls all the nation's oil. Who is at the top and who is at the bottom are sometimes an accident of history and sometimes the result of great brutality. Either way, it is difficult to persuade those at the top to cede one iota of control or assets to anyone below, as evidenced by the extreme reluctance of the wealthy to pay even the same percentage of taxes as those beneath them in the social order. 

There have been attempts to create societies in which there is no class system, but none of them has worked. Pure communism and pure socialism are fine as ideas, but once they are implemented, human nature ensures they will not work. The benefit of this is civilization, to be sure, but the down side is the ugliness that a class system inevitably engenders. If we could figure out a way to be just as motivated to build, invent, and improve, without stomping on others in the process, it would be a much better world.