It is a curious thing, Harry, but perhaps those who are best suited to power are those who have never sought it. Those who, like you, have leadership thrust upon them, and take up the mantle because they must, and find to their own surprise that they wear it well.

This quote means that those people who are best suited for leadership or positions of power are those who don’t necessarily seek out the responsibility—in other words, those who don’t necessarily actively pursue power. These people instead find that leadership has been thrust upon them—for instance, someone who has to step up and lead a team—and that person often discovers that they handle the situation quite well.