Supreme court justices are appointed for life (until they die or choose to retire). discuss the pros and cons of this policy.

The Supreme Court requires a large amount of domain knowledge that takes years upon years to acquire.  By letting a supreme court judge stay indefinitely, it lets the judges become subject matter experts and stay consistent in their rulings, which gives the perception that our highest court is predictable, knowledgeable and fair.  

On the other hand, because the supreme court has become very politicized, this means that the party that is in charge when a new justice is appointed gets a disproportionate amount of control over the rule of law for an indefinite period of time.