Article Two of the United States Constitution
Also, just by asking Congress, the President shows that he respects them as equals, and is not more powerful or important than they are, like a King would be.  One scholar explains: "The Recommendation Clause empowers the President to represent the people before Congress, by recommending measures for the reform of government, for the general welfare, or for the redress of grievances [for problems to be fixed]."p. 43