Why must we fail OR why is failure so important?
Muscle growth, particularly new muscle growth, is a defensive reaction of the body to the stress of exercise (going to muscle failure during a set), and the body must literally be forced to see necessity of adding more strength and size. This sentence totally explains why we train SO hard. Simply doing what you are capable of sends a signal to the body that there is no reason to change. Trying to do what is momentarily impossible (the next rep that you can't complete) sends a signal to the muscle that the status quo is unacceptable. Given time (rest between workouts), the muscle will change.
Weekly Quote: If you like exercise, chances are you're doing it wrong!
AS ALWAYS - GET STRONG!