From The Most Important Thing:
The choice between offense and defense investing should be based on how much the investor believes is within his or her control. In my view, investing entails a lot that isn't.
Professional tennis players can be quite sure that if they do A, B, C and D with their feet, body, arms and racquet, the ball will do E just about every time; there are relatively few random variables at work. But investing is full of bad bounces and unanticipated developments, and the dimensions of the court and the height of the net change all the time. The workings of economies and markets are highly imprecise and variable, and the thinking and behavior of the other players constantly alter the environment. Even if you do everything right, other investors can ignore your favorite stock; management can squander the company’s opportunities; government can change the rules; or nature can serve up a catastrophe.
So much is within the control of professional tennis players that they really should go for winners. And they’d better, since if they serve up easy balls, their opponents will hit winners of their own and take points. In contrast, investment results are only partly within the investors’ control, and investors can make good money—and outlast their opponents—without trying tough shots.
The bottom line is that even highly skilled investors can be guilty of mis-hits, and the overaggressive shot can easily lose them the match. Thus, defense—significant emphasis on keeping things from going wrong—is an important part of every great investor’s game.