I've honestly never understood why senators have 6-year terms. Is there any other position in the United States with that term length? 4-year terms seems to be the most standard.
The Senate is supposed to have slow turnover, its supposed to be the least affected by the passing whims of the electorate. Thats that the House is for, with its short 2 year terms. By staggering the election of Senators so that only one third of the body is up for re-election every two years it insures continuity. It makes for a simple progression, the House has a 2 year turnover, the Presidency 4 years, and the Senate 6 years.