Why are US Supreme Court justices appointed for life?

Part of the debate over rights in the 18th century involved the prerogative of kings to remove and appoint judges upon their ascension to the throne. Liberal thinkers believed that lifetime appointments would scale back the power of the king, and therefore represented social progress. If a judge was sure of his seat, he could vote according to his own judgment, despite the wishes of the king. The Whigs in Britain actually won this right, though whether it really served their cause or their government is anyone's guess.

The writers of the American Constitution probably saw lifetime appointments in the same light as other thinkers of the era---i.e., a way to limit executive strength. But lifetime appointments don't actually limit executive strength, they just distribute it over time. Doubts have been voiced by conservatives and liberals for centuries about this practice---the most obvious being that judges may, through age, become less capable of perceiving every aspect of a legal argument.

According to Federalist paper number 78, the Judicial branch of government is intended to be the weakest branch of government. This is because it was meant that the judiciaries strictly upheld the purposes of the Constitution; therefore, individual ideologies did not come into effect. Federalists 78 intended that the Judicial branch of government be independent of the other two branches.

The purpose of having supreme court judges rule for life-terms is so that the judiciaries can be left to do their business with no threat of losing their jobs. If they could be easily thrown out of government, they would could have their decisions swayed a certain way, and no longer be impartial and independent.

Government is based on a system of checks and balances, because the other two branches of government are up for reelection, and the judicial branch isn't, the judicial branch doesn't have to worry about reflecting public opinion. It then balances out the other two branches of government who may have more immediate goals during their short terms.

Federal Judges are appointed for life to insulate them from popular opinion. They are never up for "re-election" and therefore do not have to concern themselves with serving the majority. This is one of the ways that the Constitution protects the rights of minorities.

If supreme court justices had term limits, it might bias their rulings based on reelection. With a lifetime appointment, they can vote their conscience without fear of reprisal.

Life terms protect justices from worrying that every decision they make could hurt their chances of continuing their career. If justices had to be re-appointed every so often, they would be more likely to vote along the ideologies of their electors than their own.

It also keeps the composition of the court fairly stable, so that a string of Democratic presidents wouldn't necessarily mean a completely liberal court, or a string of Republican presidents wouldn't necessarily mean a completely conservative court, although the majority of the current court is extremely conservative.

Because they wanted the judiciary to be independent of the political process so that judges would make rulings based on the law rather on evanescent political calculations.