The FAA defines learning as "a change in behavior resulting from experience," which means flight instructors need to be keen observers of human behavior in order to determine if learning has occurred. Much of a flight instructor's job involves on-going evaluation of a pilot's behavior in several dimensions and then providing constructive, critical feedback to the pilot. How they talk on the radio, how they use aircraft systems, and how well they comply with standard procedures for aircraft operation all need to be carefully observed. Having instructed for several years, I've come to the conclusion that good pilots, like most good all-around human beings, tend to be good at two very difficult, yet important tasks.
The first skill is freely admitting that you have made an error. Most of us don't want to make mistakes and each of us tend to be our own harshest critic. Pilots in particular tend to be compulsive about, and identify strongly with, the quality of their performance. When a mistake is made and recognized, this understandably makes us uncomfortable and may create cognitive dissonance. But before a error can be corrected, we have to recognize that a mistake has been made. Flight instructors need to be good diplomats, because many pilots simply don't want to recognize their errors or they want to explain them away.
Denial and rationalization are two traits that do not serve a pilot well, because they cut you off from the very experience that can improve your performance. It's unlikely that you can change your behavior (learn) if you can't accept what just happened. After taking eight different FAA knowledge tests, seven FAA check rides, numerous proficiency and simulator checks, and countless FBO aircraft check outs, I think I have finally learned to readily and freely admit when I've made a mistake while piloting an aircraft.
The sooner a pilot can learn the skill of admitting they have screwed up, forgot, or overlooked something, the safer they will be. A good instructor doesn't just point out mistakes and faults, they also offer insight into how the problem occurred and strategies the pilot can use to improve their performance.
For student pilots and pilots training for a new certificate or rating, being able to quickly learn from your mistakes will save you time and money. And if you need any evidence that saving time and money are important, look no further than the ever rising price of avgas. Once you've admitted to an error, to yourself as well as to others, you now have the opportunity to learn (change your behavior).
I strive to temper the feedback I give to pilots by emphasizing the difference between minor errors and mistakes that could kill you. Most of the time, pilots make benign mistakes that, in and of themselves, are not dangerous. The problem is that a chain of mistakes, even if they are benign when considered separately, can compound and add up to something dangerous, maybe even deadly.
So the goal of a good pilot is not to avoid making any mistakes, but to recognize and correct the ones that s/he makes. As Hamish said so eloquently, expecting perfection of yourself might turn out to be the most deadly form of arrogance.