"Every man has but one destiny," Vito said, more than once. There are several reasons why Michael would have made a better Don than Sonny and Fredo. He didn't want any part of his family at first. And, even after he stopped being a civilian by whacking Mac and Sol, he could have left the family at any stage. But, he really wanted to be the Don once he took the plunge--it was, you might say, his destiny.