Why do Supreme Court justices sometimes disagree on the meaning the constitution?

Answer 1

The purpose of the Supreme Court (in the US) is to exercise oversight on the executive and legislative branches. In short, if a particular law or executive action is problematic enough to work its way up through the legal system to be reviewed by the supreme court, the supreme court will rule on whether it should stand or be reversed based on its compatibility with the US Constitution. This requires a deep and sophisticated reading of the meaning of the constitution, and there are differing opinions on how the constitution should be read. The constitution itself is fairly schematic on most issues, listing out some general principles that were deemed important by the founding fathers but not going into great detail, because the founding fathers knew that the legal system would have to evolve over time. For instance, the 2nd amendment (literally speaking) only grants citizens the right to bear arms when part of an organized militia. For the founding fathers this probably meant citizens in a group like the National Guard - para-military organizations which could be called into actual military service by the government at need. Narrow interpretations of the constitution would restrict gun-ownership to just those conditions; broad interpretations, however, have generally held that this amendment grants gun-ownership rights to citizens in general (originally as an internal defense against tyranny, now more as a personal defense against criminality). the intentions of the constitution on this point, and the correct interpretations that should be made, are still hotly debated in legal circles.

Answer 2

The interpretations change because some justices are replaced by people who hold different views. The court's decisions are reached by vote of the justices. There are nine justices, so if there is a close vote, the final vote (by the Chief Justice) will be the deciding vote; however, if just one of the other justices changes his/her mind before the final vote, the matter will be decided by that one justice: the vote would change from 4 to 4 plus the Chief Justice to 3 to 5, so the Chief Justice's vote would not change the decision either way.