Why did the founding fathers want the constitution to be flexible?

When our founding fathers created the Constitution they realized that any document meant to frame a government needed flexibility. They wanted the Constitution to be able to stand for generation after generation. In recognizing this they incorporated two important features:

In this clause our founding fathers state that congress may pass all laws necessary and proper. This then allows for a loose interpretation of the constitution and allows constitutional flexibility. Thomas Jefferson was very much opposed to this clause and as you can well imagine Alexander Hamilton was it's author. Some examples of the elastic clause in action include Hamilton's creation of the National Bank and Jefferson's purchase of the Louisiana Territory from Napoleon. The fact that Jefferson actually used the elastic clause is an irony not lost on either Hamilton or Jefferson.

Here is the original text:

To make all Laws which shall be necessary and proper for carrying into Execution the foregoing Powers, and all other Powers vested by this Constitution in the Government of the United States, or in any Department or Officer thereof.