He explained this in his autobiography:
I declined to aosevdopt the view that what was imperatively necessary for the Nation could not be done by the President unless he could find some specific authorization to do it. My belief was that it was not only his right but his duty to do anything that the needs of the Nation demanded unless such action was forbidden by the Constitution or by the laws. Under this interpretation of executive power I did and caused to be done many things not previously done by the President and the heads of the departments. I did not usurp power, but I did greatly broaden the use of executive power. In other words, I acted for the public welfare, I acted for the common well-being of all our people, whenever and in whatever manner was necessary, unless prevented by direct constitutional or legislative prohibition.William Howard Taft referred to this as the Stewardship Theory. Not only must a president do what the Constitution commands him to do, he must also push forth his agenda if he believes it serves the general welfare. The exception here is "unless he is expressly forbidden not do it" by the Constitution.
The problem with this is that many future progressive presidents took this a step further, and made it so their own personal agenda was prioritized above and beyond the Constitution. When this started to happen was when chaos in our nation started to ensue.