More recent rantings, (from a slightly different angle) on this topic can be found here and here.
There is definitely a fine line between training smart while pushing yourself very hard, and overdoing it. It's not always easy to know where that line is. It gets very blurred. How much is enough? If you're a competitive driven person, this is a particulary difficult issue. You demand the very highest output from yourself when you're training hard. It doesn't feel right to end a set prematurely or to give anything less than everything you've got, during every workout. Ultimately I think it comes down to the fact that your psychological self-worth is directly tied to your performance.
Knowing where productivity occurs and where it ends is difficult. There is a point in a workout where any additional work is not productive, and is perhaps counter-productive, degrading your ability to recover and come back stronger. It's the law of diminishing returns. More is simply not always better. Quality over quantity. We all have heard these truisms, but when you're ego is on the line and everyone is watching, are you really gonna remember that? Not likely. You're gonna push on. It is my opinion that ego can be one of the single most destructive forces, potentially wrecking your career, sidelining you with injury or chronic overtraining.
On the other hand, I have not seen many elite level performances coming from people without a big ego, on some level or other. Perhaps some are just better at hiding it or controlling it than others are!
As a coach, I would much rather "rein in" the overachiever, than "whip up" the unwilling. I'd rather teach the art of restraint to a firebreather, than I would attempt to push someone into the fire, teach them to embrace pain and intensity, when they would prefer to avoid it.