What are the main reasons for changing the constitution?

The main reasons for changing the Constitution was that before the Constitution there was the Articles of Confederation that had only lasted for 7 years before. In the original Constitution they had still had slavery as an amendment and they started adding on to these after the 11th amendment such as when you turn 18 you can vote. So in conclusion main reasons would have to revise goverment plans and to give power to the people as well.