Article 2, Sec. 3 of the Constitution charges the President "shall take care that the Laws be faithfully executed." It doesn't say that he "should" execute the laws of the United States; it uses the imperative "shall."
Nor, does the Constitution say that the President can pick and choose to enforce some of the laws, or just the ones he likes.
Nor, does the Constitution give the President the authority to create new laws. Article 1, Sec. 1 is clear on that point; "All legislative Powers herein granted shall be vested in a Congress of the United States which shall consist of a Senate and House of Representatives."
But for Barack Obama the limits of the Constitution matter very little; particularly so the separation of powers that the Founders crafted as a check-and-balance against an unhealthy concentration of power in one branch of the government. In fact, Obama has publicly derided the Constitution as a "charter of negative liberties" and pledged to work to "break free from the essential constraints placed by the Founding Fathers."
In a recent column Charles Krauthammer addressed the issue of a President who "writes his own laws." Krauthammer affixes the title befitting of the President's lawless nature: "Caudillo in Chief." After a litany of grievances as examples document his assertions, Krauthammer concludes:
Such gross executive usurpation disdains the Constitution. It mocks the separation of powers. And most consequentially, it introduces a fatal instability into law itself. If the law is not what is plainly written, but is whatever the president and his agents decide, what's left of the law?
What's the point of the whole legislative process — of crafting various provisions through give-and-take negotiation — if you cannot rely on the fixity of the final product, on the assurance that the provisions bargained for by both sides will be carried out?
Consider immigration reform. The essence of any deal would be legalization in return for strict border enforcement. If some such legislative compromise is struck, what confidence can anyone have in it — if the president can unilaterally alter what he signs?
Yet this president is not only untroubled by what he's doing, but open and rather proud. As he tells cheering crowds on his never-ending campaign-style tours: I am going to do X — and I'm not going to wait for Congress.