"Man is born free; and everywhere he is in chains. One thinks himself the master of others, and still remains a greater slave than they. How did this change come about? I do not know. What can make it legitimate? That question I think I can answer."
To Rousseau, everyone was born independant and with unlimited opportunities. But also with unlimited risks. This, he thinks, is why people form societies... each person surrenders some freedom willingly in exchange for the greater safety provided, at least from their fellow-man, if not from nature.
Here is where many governments take a left turn. Presumably a government enforces the will of the people... but many do not. What is the difference between a bully forcing you to do something and a society forcing you to do something? Arguably very little. If you have no say in the rules, then you are just a victim of them. A slave. You are bound by codes and conventions whether you like them or not.
The only good government, therefore, was an almost purely democratic one. One where the people made the laws and everyone had some influence, even against the tyranny of the majority.
This also is the answer to the conundrum Rousseau poses in his inttroduction: A master is a greater slave then the slaves he owns because the slaves are free to act in any manner that they can get away with, up to and including killing their master. The master, on the other hand, must create and enforce a social bond or be destroyed.
And so, perhaps, it is with every man. We are born with limitless potential, yet, willingly or otherwise, we are shackled by the rules that are placed upon us to ensure the safety of other people. Hope that helps!