Elevator Behavior

Why do people so often feel the need to press a button that has already been pressed? This happens especially often on the way down in an elevator. If I am in the elevator going down, of course I pressed Lobby! Oh, and it's lit up already too. This scenario also occurs often when waiting for an elevator. In this case, someone always makes sure to re-press the up or down button as if that might make the elevator come faster.