The Constitution is interpreted and amended according to the needs of the times and because early leaders established a reverence for the Constitution that makes it an indispensable part of America's political heritage. Leaders advocate laws by imbuing passages of the Constitution with new meaning. Despite constant metamorphoses, the political norm is always to defend policy proposals by resorting to the Constitution.
The Constitution's style and structure facilitate new interpretations. It is a relatively sparse document, much shorter than those of many other nations. The written details surrounding the roles of the executive and judicial branches are especially vague and lacking in detail. Thus, modern political realities, not enshrined instructions, determine most of the specific workings of government.
The judicial branch also has a hand in keeping the Constitution vibrant. The principle of judicial review allows the Supreme Court to interpret the Constitution in deciding whether public policy is constitutional or unconstitutional. This practice has often led to the court finding rights and functions of government far beyond what the actual written text specifies. These trends combine to keep the Constitution relevant.
Another important factor for the Constitution's longevity is the immense reverence Americans have for it. The Constitution itself requires all federal officers to take an oath to uphold it. From the onset, political leaders honored this allegiance and debated political questions on the matter of constitutionality. In modern America, citizens still view the Constitution as something sacrosanct and inviolable. Opposing parties do not claim that the Constitution is wrong, but that their opponent's interpretation of the Constitution is wrong.