Why do you think President Roosevelt said that the Constitution " ... ought to be read and read again?"

He said this because president Roosevelt thought that when leading a country, and even if you're not leading it and want to be a law-abiding citizen, reading what the founding fathers have written would be good as they had a clear vision on how they wanted this country to look like. Because of this, we should strive to seek wisdom from the constitution and look for inspration in it.