The 17th Amendment changed Madison's wording in the Constitution, and changed it so that senator's were directly voted in by the people. The reason for the change was that progressives, like Woodrow Wilson, believed senators, since they were representatives of the people and not the states, would be more likely to vote for certain things that were not in the best interest of the states, and may be in the best interests of special interest groups and lobbyists. In essence, the 17th amendment turned senators into lobbyists for Washington.
A good example is the health care program that just passed through Congress. If the senators were representatives for the states, they never would have voted for it because that law does not have what's best for the states in mind. This is exactly how Woodrow Wilson and the progressives wanted it. They knew they would never get their radical, socialist agenda passed without the 17th amendment, or without the support of Washington.
Madison new checks and balances were needed, and having the senators be representatives of the state was one of them. When the people don't support something, which they did not support the Obama Health care Bill, the progressivesk new they could still get it passed if the senators that were elected did not represent the states.
This is exactly how much of the progressive agenda was passed, however unpopular. The rest of it is made law by radicals in the judicial system who make laws from the bench that would never be passed through the legislature. A good example of this is the 1972 Rowe Versus Wade ruling that essentially made abortion legal from the bench.
The 17th amendment basically trampled on the 10th amendment, and therefore trampled on state rights.