After the Cavaliers unexpected early exit of the playoffs, one thing became abundantly clear: Lebron isn't going to win as long as he's a Cavalier.
While the Cavs will do whatever it takes to keep Lebron, they've acted like that girlfriend who could tell that you're about to dump her, so she does anything and everything she can to make things work, but in the process she just ends up smothering you. While you do like the extra "action" you're getting, and you can appreciate the nice deeds, her insecurity is annoying (not to mention kind of pathetic), and the two of you are fundamentally flawed as a couple, and now, it's almost impossible to reconcile because there are aspects of both of you that make it so you won't ever be able to coincide together AND have a good thing going.
Cleveland doesn't have enough young talent, they don't have enough cap space to sign both Lebron and the kind of #2 that he really needs to get the Cavs over the top, etc., while Lebron doesn't appear to have the patented "MJ: do-whatever-it-takes-to-win", or "the cold-blooded, soul-crushing, demoralizing factor where in a tight game, you know there's nothing you can do; my team is going to score and win the game" abilities.
Not to mention, if Cleveland was to "start over" per se, Lebron makes them too good to get anyone decent in the lottery, and Cleveland hasn't proven that they can find gems in the draft, i.e. most of their roster has been accumulated through free agency and trades.
Miami on the other hand is a bad fit too. Yes, they do have DWade and Michael Beasley, but almost everything else is coming off the books next year.
They will be a bunch of scrubs, a decent developing role player, and DWade and Lebron. And Bill Simmons put it best when he said "he [Lebron] doesn't want to be perceived as riding someone else's coattails", when referring to him paired up with DWade, who has already won a championship and thus, proven himself. And even if Lebron were to go to Miami, any team with a good front line would kill them. Miami is going to be fundamentally flawed until a) they can lure a good, big 4 or 5 in free agency, a la Joakim Noah, Chris Bosh, etc, or Wade is going to have leave or get injured for most of a season so the Heat can lose enough games to get some good pieces in the draft.