"At the Constitutional Convention in 1787, the Founders were guided by, among other powerful ideas, the fear of the potentially tyrannical, harmful powers of a democratic majority. The proposal that United States Senators should be elected by state legislatures, rather than democratic majorities, was inspired by this fear. . . ."
To understand why the U.S. Senate was originally select by state legislatures and why this changed with the passage of the 17th Amendment, you cannot go wrong reading the rest of the blog post.