Because most who want the throne are not fit to have it. They are attracted to the power and do not understand what it truly means to wear a crown.

[Honestly, she would give Daenerys credit for that. She cared more for her people than most monarchs she had met. But she was blood thirsty and a threat to the Starks and the North as a whole, and Sansa did not stand for it.]

For all three of us, you, Robb, me... We were chosen by the people. Same with Bran. There was this line I read here, some time ago. Some are born great, some achieve greatness, and some have greatness thrust upon them. It makes me think of that, somehow.