The average age of Supreme Court Justices is 69 years. This fact alone brings into question whether these judges should be appointed for a lifetime. For, judges of this age can easily be in poor health mentally and/or physically, a fact which could affect their decision-making abilities or the outcome by their absence on a case. This is one reason no one should be on the Supreme Court for a lifetime.
Another reason that a Supreme Court Justice should not sit for a lifetime is the fact that some candidates have misrepresented themselves during the confirmation process, a fact which is evidenced in their later decisions as Justices. A case in point is Sandra Day O'Connor who was appointed by President George H. Bush, who felt that she was in accord with his conservative views. In reality, she was not at all of a conservative view as proven in her decisions while on the Supreme Court.
A third reason that a Supreme Court Justice should not serve all his/her life is one of the reasons that a President is not allowed to serve more than eight years. If a president has an extremely radical agenda--right or left--his/her term can be ended after 8 years. However, the nation must often suffer for possibly 20+ years under such a Justice.
Because Supreme Court Justices carry extreme power and greatly affect the nation in interpretation of the Bill of Rights, etc., their terms should be no more than eight years.
The Framers did intend those that serve on in the Judicial Branch to serve in that capacity for life. Naturally, the legislative branch can impeach a Justice should there be any wrong doing or breaking of the law. Yet, in the larger sense the Justices are meant to serve for life. There are a couple of potential reasons for this . The first would be that the Executive and Legislative Branch members serve at the behest of the public. Being dependent on the principle of popular soverignty, these individuals are policy makers who are responsive to the needs and demands of the public. This is different in the Judicial Branch, who is seen as the guardians of the Constitution. This capacity is not contingent on the public, as the Justices are not in the function of making or passing laws, but rather assessing their Constitutionality (whether or not the law correctly applies and interprets the Constitution.) The ability to interpret Constitutionality checks the other two branches and is not something that is dependent on the public. With this in mind, this branch was seen as something that can enjoy a lifetime of service, not having to worry about reelection or the demands of popular soverignty. The idea of having the justices serve for life would testify to the notion that the Constitution is something that is permanent, ever lasting and will not wither. It only makes sense that the branch whose primary responsibility is to guard it consists of individuals who are about as everlasting as possible. In these ways, the Framers' logic makes sense.