The answer is inferred in the last paragraph. When it says "each man, in giving his freedom to all, gives his freedom to no one" means that if we all have the power of decision we all have to consent to a specific outcome or solution together. No one has more power than the otherone. If our wishes align with the desires of the majority of people we have the possibility of those demands or expectations to be fulfilled. Because of that, is in the contract with others where we can maintain, as better as possible, our personal freedom.
On the contrary, if there is just one person in power, for example, the king, his will would be the one being made without taking into account what the subordinates might think or want. In monarchies, if the king ordered someone to do something, they had to do it regardless of what they preferred. In those cases, individual freedom was completly coerced.
To sum up, the power to set and change the rules of society is dispersed among all people in a society ruled by the social contract idea. By sharing the power among everyone the individual maintains its freedom better than when the power is held by a single ruler that withholds all of it.
