Desire. As has been mentioned, you talk to 10 basketball trainers, and 95% of what they harp on is offensive moves, shots, drives to the basket, sealing off in the low post, etc etc etc. None of these people will show you clips on their site about defensive drills. And from what I've seen, it's an AAU disease. Everyone wants to dunk or hit the three.
I play pick up with a bunch of BoingVert steroidal maniacs... they spend all their down time practicing tomahawks, 30 foot and longer threes, and alley oops. All energy is spent on offense. I'm 54 and not one of them bothers to play any defense.
Defense is simple; take charge of the offensive possession. Make the ball react to you, don't wait to react to the ball. That simple concept freaks most players out. Get a hand in their face, you don't need to leave your feet on every pump fake and move to the basket. Stick a hand in their face and it normally disrupts their shot. It also takes a tremendous amount of energy to do all this consistently. The few players I've seen that exert some energy on defense are constantly looking for the noisy block into the fourteenth row. Like boxing out, following your shot, and constantly moving without the ball, defense is the boring, grind it out part of the game.
A freak athlete can be a great defender. It's a choice though. MJ used to say, "all that dunking takes a lot of energy".