LeBron left Cleveland for someplace where he thought it would be easy to win. He left Miami when it looked to be getting tough for someplace where he thought it would be easier to win.
There really is very little evidence for this. Winning with a much younger and less experienced supporting cast is always harder, not to mention the greatly raised expectations trying to win one for your hometown team.
Based on what has come out in the last year, it seems LeBron went back to the Cavs because
(1) He always wanted to return there eventually and try to win a title for Cleveland
(2) Things were going downhill in Miami sooner than expected
(3) LeBron wanted to have much more control over how the team was run
He acknowledged in his announcement that the Cavs would be a bit of a project because of how young the team was. Obviously some of that was PR spin, but I think to some extent that was genuine.
Heading into this season, I doubt LeBron necessarily expected to contend. The mid-season trades really worked out for them and the Eastern Conference was even more of a joke than usual.
To even have a chance at a ring, all things considered, is gravy for Bron, at least for this year. He's gonna have at least 4 or 5 more chances to make the Finals with the Cavs. But that's not how it'll be talked about if the Cavs lose.