Our Constitution was not a proclamation written by a convention. It was a negotiated contract for uniting thirteen sovereign independent states. Nothing like that had ever been done voluntarily, and few nations have matched it in two hundred years, even with the use of force.
Although republican forms of government were almost entirely supplanted by despotism in various guises for two thousand years after Julius Caesar, this neglect of democracy was seldom tinged with criticism of majority rule until the Eighteenth century Enlightenment, when majority rule became more of a possibility. The disturbing discovery that a majority could abuse a minority just as thoroughly as a king might, began to haunt the thoughts and writings of the opponents of despotism. Even the proponents of Vox populi, vox dei est ("The voice of the people is the voice of God") had difficulty threading a way between accepting the tyranny of the multitude, and rejecting majority rule. It is therefore a pity the secrecy of the American Constitutional Convention prevents us from knowing whether the gradations of majority decisions which emerge from a reading of the final document, was the product of a single mind or an evolving consensus. As Alexander Hamilton pointed out in Federalist No. 84 and Federalist No. 85, the language of the Constitution is riddled with exceptions to generalizations which the Convention had reached by majority vote:
Independent of those which relate to the structure of government, we find the following: Article 1, section 3 clause 7-- 'Judgment in cases of impeachment shall not extend further than to removal from office, and disqualification to hold and enjoy any office of honor, trust or profit under the United States, but the party convicted shall, nevertheless, be liable to indictment, trial, judgment or punishment according to the law.' Section 9, of the same Article, clause 2 -- 'The privilege of the writ of habeas corpus shall not be suspended, unless when in cases of rebellion or invasion the public safety may require it.' Clause 3 -- 'No bill of attainder' etc., etc."
Hamilton was of course arguing that no Bill of Rights was necessary, because ample latitude had been provided for the writers of rules to make nuanced exceptions when the rules are created, or else amended afterwards by the elected legislatures. Nevertheless, a Bill of Rights became the first item of business of the new Congress under the Constitution. Even in the age of the Enlightenment, despotism was still so much the rule that most people could only think in terms of an occasional exception for egregious circumstances. In a sense, this limitation of Royal or other prerogatives began with the Magna Charta of the 12th century, and many Englishmen believed that a Constitution merely exists in order to spell out broad limitations of sovereignty. Many opponents of the Constitution voiced the opinion that a monopoly of the armed forces essentially confers unlimited sovereignty on whatever entity has such military power, so what was left to define were the handful of limitations which rebels would be willing to die to preserve. Many otherwise unexplained provisions of the Constitution are merely implicit statements of this intent. We are fortunate that Hamilton went on to illustrate the nature of the concern about one feature of the internal debate about amendments to the Constitution. There existed an uneasiness that nine states might conspire to surprise the others with an amendment for reasons only known among themselves. Therefore, the Constitution requires a 2/3 vote to introduce an amendment, but a 3/4 vote of the states to ratify it. In at least one uninvolved state therefore, the people outside the conspiracy must have a chance to examine and debate the issue. One must credit at least one doubter among the Founding Fathers with imagining a sly strategy which has never been attempted, and blocking it without accusing anyone of even imagining it.
Unanimous consent is of course qualitatively different from majority rule. The requirement for unanimous consent had proved to paralyze the Articles of Confederation. It implied that no legislation would ever seem so beneficial to a preponderance of the states, that no matter how trivial the injury to a single remaining state, it must not be allowed to pass. Any rule short of unanimity implies that some segment of the population must occasionally suffer some degree of injury in order to serve the best interests of the others. In practice, of course, it means compromise, one regular feature of which is honoring the implicit debt the others owe to the victim state, on later legislation where interests lie in other directions. That comes in various degrees of severity, so in some cases it may be necessary to insist on a supermajority for enactment, perhaps two thirds, or three fourths. It is no more a shame to defeat a supermajority by one vote, than it is to get one vote more than half. And halves come in different guises, half of those present and voting, or half of the eligible membership of the whole body. Furthermore, the majorities or supermajorities may come in different venues: a bill must pass both the House and the Senate, and then be signed by the President, who is allowed the face-saving escape of a pocket veto when Congress adjourns. Or combining veto power, combined with a second chance to enlist a vote to over-ride that veto. In the case of Amendments to the Constitution, all of the participating branches of government are given a chance to defeat it. In the case of Judicial actions, decisions are stratified within the Judicial Branch. A district court establishes the facts, an appeal court judges the application of the law, and the Supreme Court can judge the constitutionality of the law. Although these last distinctions do not appear in the Constitution itself, they do reflect the Constitution's intent to introduce subtlety into what is, in general, majority rule. With all of these variants of strength of a majority or minority, an opportunity is created to vary the degree of pain which a majority might inflict on a minority, or else the damage a minority can inflict on the best interests of the community as a whole. It's called flexibility.