Rome had a fairly advanced culture and government. It was well-organized, with methods for taxation and rule by proxy. The government also encouraged innovation. This made it possible for Rome to expand farther and farther.
The value of being a Roman citizen was something that was famous at the time, and we can see that the development of the Roman empire was in part based on this social concept. The protection of citizens of Rome and the extra status given to Roman citizens in the empire meant that the rulers of Rome had to at least consider the perspective of the citizens in their decisions.
Two social ideas, which were tied to a political idea, was (1) the value of the participation of all citizens in government and (2) the protection and fair treatment of all citizens. Because of these social ideas, even plebeians had a say in their government (an indirect say) and the expectation of fair treatment in accord with human dignity.