I'm new here so forgive me if this has been asked, and if it has, just point me towards that thread and I'll take this one down...

So I loved the first 2 movies and I have very few complaints, but my big question is how the hell did Michael become boss in the first place? I got the impression that he wasn't even involved in the business and he was also really young. How does he pop out of nowhere and become the don? Isn't that like tony soprano making AJ the boss?