If you've ever ridden a poorly built frame that feels like a wet noodle, you'd know the answer.

Now for my serious answer, I will say that a frame could possibly be "too stiff", especially in endurance events. If your body fatigues from road bumps, you also lose energy. The trick is finding a bike that is stiff enough to transfer your energy to the drive train efficiently, yet forgiving enough to keep you from losing concentration because of your sore @$$.

Oh, and celeste paint makes frames stiffer than red