The role of the presidency has changed in very major ways since 1789. In the originial Constitution, the president was seen as very much the junior partner to the legislative branch. This can be seen in the fact that early presidents believed that what they were supposed to do was to simply carry out the laws passed by Congress.
Over the years, partly because of the expanding role of the government, the role of the presidency has exploded. We now see the President as the one leader of the nation who is supposed to make sure that everything is okay. If the economy does badly, we blame the President. If our military does well in a given operation we give credit to the President. The President is now seen as the person who is supposed to dominate and set the national agenda and who is responsible for the overall well-being of the country. This is far beyond what the Founders expected the presidency to be.