Supreme Court Justices in the United States serve "during good Behaviour," according to the U.S. Constitution. This has been interpreted to mean that they hold their offices for life unless they are impeached and convicted by Congress, resign, or retire. The reason for lifetime appointments is to maintain judicial independence, a crucial aspect of the separation of powers principle that's foundational to the U.S. system of government. By not being subject to re-election, justices are theoretically insulated from political pressure and can make decisions based on their understanding of the law and the Constitution, not on what is politically popular or would get them re-elected.