After Franklin D. Roosevelt's 4-term Presidency, the 22nd Amendment was proposed and ratified, limiting all future Presidents to no more than two full terms, or eight years in office. This seems reasonable for a republic such as the United States, so that we, the citizens, would not have to live under the same Executive for possibly twenty years or even more, as there are many countries to which, once a particular personage rises to their equivalency of the Presidency, do not ever leave office, unless they are overthrown, or die, while in office.
In regards to the judicial branch, judges are appointed to stay on the bench, while maintaining "good behavior", which basically equates to lifetime appointments. This may or may not be a good idea, but at least it is settled policy, whereas for congressmen, both for the Senate as well as for the House of Representatives, there are no terms limits, whatsoever, and those that are members of such, have proven again and again, that once in office, they have no intention of ever leaving it. In fact, there are at least 50 members in the history of Congress that have served during their tenure uninterrupted terms of 40 years or more while in the Senate or Congress of the United States of America. This length of service is far too long for those that are in theory, public servants, to the people, and should not be permitted, so consequently we need a Constitutional Amendment to this effect.
It doesn't seem fair to the public, that the legislative branch of government, elects members of such, that once elected, have a strong desire, often realized, of staying in office, for year after year after year. These elected men and women are the actual lawmakers of America, and rather than being seen as servants of the people, are in fact, the masters of the people. In point of fact, the only way to pry back cronyism, favoritism, corruption, and ilk of this sort, is to have as a matter of policy, term limits, to which no member of the Senate should be able to be elected to any more than two terms, or a total of twelve years, and no member of the Congress should be able to be elected to any more than four terms, or a total of eight years.
For those that argue against Congressional term limits, with the reasoning, that each election cycle gives the people a choice, which seems both democratic and reasonable, this is countered by the fact that incumbents by virtue of being in office in the first place, have enormous advantages over any other challengers either from within the party or from the opposing party, because specifically, the power and money behind politicians is only interested in backing winners, and the smart money almost always backs the person already in office, so that business can be conducted as usual.
Further to the most basic of points, the only real reason why any Senator or Congressman would run for election and hold office term after term after term, is because the benefits, the power, the influence, the money, the prestige, is so enticing that once in, they do not ever want to get out.