Originally Posted by Quicksilver

Simple:
The key word here is "overconfidence" Overconfidence leads to carelessness, carelessness leads to mistakes and accidents, and in some cases death.

For Example:

WELL-PLACED confidence is a good and useful thing, but both secular and sacred history warn us against the snare of overconfidence. Ancient Babylon was overconfident, only to fall to the armies of Cyrus. And Victor Hugo, the noted French writer and historian of the last century, tells of the overconfidence that Napoleon displayed before the battle of Waterloo:

His overconfidence caused him to believe that victory truly was within his grasp. But history was to record the opposite. So much so, in fact, that “Waterloo” has become proverbial for “a decisive or disastrous defeat or reversal.”