The Framers of the Constitution wanted the House to be "nearer" to the people and for the Senate to be "farther" from the people. They wanted the House to really closely follow what the people wanted while the Senate acted like a brake on what the people wanted. This is why they have different lengths of term.
The 2 year term for the House is meant to ensure that they Representatives will always represent the will of the people (because they are elected frequently). The Senators' 6 year terms were meant to ensure that they would be more detached from what the people wanted.
So the longer terms for Senators are meant to make them behave less as the people want and the shorter terms for Representatives are meant to make them behave more as the people want.