From Coding Horror:
There are a bunch of ways to restate the paradox of the active user. Cooper calls it perpetual intermediacy. I think the easiest way to explain it is this: every user lies. Instead of asking users if they love your software-- of course they love your software, it'd be rude to tell the people responsible just how mind-numbingly awful it really is-- do what Gregory House does. Observe whether or not they use the software, and how they use it. Rely on that behavioral data to design your software, and not the lies of your users, however well intentioned they may be.
One of those frustrating moments when I think nothing I say will make this clear, unless it's already clear, in which case adding anything would be pointless and, possibly insulting.