I can make a pretty good case.
First, out of the Western Conference. He will have to beat one really good team to get to the finals and Lebron isn't getting any younger. Golden State and SAS are going to be good for awhile and Minnesota and Portland are up and coming. LAC will be contenders as long as Paul is still able to get it done and the Lakers will probably attract some free agents at some point. The East has Miami on decline, Atlanta who just traded Teague, Horford who might leave and Korver who is getting older. The East could be there for the taking in a few years.
Secondly, Durant does seem to like the fans, the town, and the basketball history of the Celtics. You never know what that means to a guy.
Pot roast. Terrance Knighton for some inexplicable reason is great friends with Durant and is recruiting him to Boston. Plus IT might be, I'm not sure. I know Smart has some connection with Durant from his OKST days. So he knows some people in the area, that might ease some of his concerns.
If Durant goes to GS or SAS, or even Miami, he is joining someone eles's team. If he goes to Boston, it immediately becomes his team. He is also a fit in Boston where he wouldn't be for the Knicks. Boston has defensive players, they need a big time scorer. Durant is a big time scorer. He doesn't have to worry about sharing like he has had to do with Russell or like he'd have to do basically anywhere else.
Boston can bring in two max guys. Yeah every team will be able to bring in, or create space to bring in a max player like Durant, but Boston can bring in Durant and a friend. Whether that friend is Horford or Kevin Love, Boston can bring in a free agent, or they have tons of future assets and good contracts to bring in a specific player if Durant wants.
Lastly, most people think Durant isn't leaving. I think he is. I don't think he wants to go back to OKC for one more year and then leave. I think he wants to pick the place where he will finish his career. From what I've heard, Russell wants to go to the West Coast and play for the Lakers. I don't think Durant is going to stick around and wait for Westbrook to leave. That would seem a bit beneath his talent. It seems to me he wants to get started on the rest of his career and Boston is a really good fit for that.
Some people would think, well Miami has a couple stars there and that would seem like a better fit. But Wade is old and breaking down. Durant is going to want to pick a team that is set up for the long run, not just next year. Lebron already ditched Miami to go hook up with some younger stars in Cleveland. I can't see Durant jumping in there.
I can't see Durant joining an already established team with Curry. I can't see him going to San Antonio to compete with Aldridge at the 4 or Kwahi at the 3. No way he joins Lebron. No way he goes to Miami with Wade breaking down, Whiteside probably leaving, and Bosh's future in serious jeopardy. He won't go to the Knicks if he likes seeing the ball on offense. I don't think he is staying in OKC, especially now that Ibaka got shipped out. Would he join the Clippers? Really? Paul isn't getting any younger and they'd have to move Blake to get him. That leaves him with an aging Paul and a center who can't shoot free throws. Houston seems like a mess and have coaching issues. They also have Harden issues.
So why not Boston? They need a scorer, he fits. They have money to bring in guys he likes. They have a great young coach that is getting mad respect from everyone, especially Popovich which carries a lot of weight. They have good young players AND some really good assets to add pieces in the future. He has friends in Boston already so he won't be alone. Seems like a really good fit to me.