It's the typical struggle between our inner virtual and self-interest. If we in a society would be governed by our inner virtue, we would voluntarily take care of one another -- your cooperation model. However, if we allow self-interest to rule, we need laws to keep all the self-interest in check and we lose freedom. Only a virtuous society can be truly free.
"No free government, or the blessing of liberty, can be preserved to any people but by a firm adherence to justice, moderation, temperance, frugality, and virtue, and by frequent recurrence to fundamental principles" -- George Mason
"Statesmen...may plan and speculate for liberty, but it is religion and morality alone which can establish the principles upon which freedom can securely stand." -- John Adams