The Constitution basically says that the Justices are given lifetime appointments as long as they are on good behavior. The Congress wanted a system and people who were unbiased and wouldn't try to please the new president, they just wanted the justices to be normal and not be influenced (the Justices, if they wanted to get reappointed, would try to please the president.). The justices are also given lifetime salaries so that Congress cannot threaten to take away that person's power if they don't vote accordingly.