The main reason I see as to why Durant would leave OKC would be because he has lost faith in the franchise. Like all greats, he wants a ring. If he feels like OKC won't really ever get over the hump, then I can see him signing somewhere else. I doubt money is much of a concern. These superstars make way more money through endorsements rather than their salaries anyway.
But honestly, his team beat the Spurs and brought the historic Warriors to game 7. They were up 3-1 at one point in the series. I don't see how he could find any real reason to leave. He doesn't strike me as a guy who'd join the Spurs or the Warriors. He feels more like a KG than a Lebron. Im sure he'd love to win one for OKC.
I think you basically stated the biggest reason right there - they're just not able to get over the hump. They were up 3-1 in the WCF and choked it away, which has always been the knock on them, especially this year.
I agree with your assessment about his personality, though. I think there's absolutely no way that he joins GS, who shouldn't really want him anyways, though I'm more worried about the Spurs.
I think the reason he'd leave is, 1) Westbrook is leaving when his contract is up and going to L.A., 2) in the West to win a title he is going to have to go through two of Golden State, San Antonio, and the L.A. Clippers to get to the finals, EVERY YEAR, 3) Minnesota and Portland might be on the rise in the West as well, 4) even if he does get to the finals, then he still has to win against the Cavs. That is one heck of a gauntlet to run for Durant the next 5 years.
Now, if he goes to the East, 1) he has the a pretty smooth road to the ECF, 2) he has to beat the Cavs who he'd have to beat anyway and one team from the WC who just ran an incredibly tough gauntlet to get to him, 3) he gets to pick the team he sees as the best fit instead of just the one that drafted him, 4) if he picks his team with assets and cap space, he can bring in the players HE wants to play with.
I don't see why he wouldn't leave other than loyalty. And of all the teams in the East, Boston seems like a really good fit for him. It is a team built on defense and just needing a scorer. That is what he does.