Exactly like Lebron with Miami. He envisioned himself playing with Wade and Bosh, hanging out, traveling together as a team, and thought it would be a lot of fun.
It can't be said enough that being around a personality like Westbrook would get really old and tiresome. He did it for 9 years (8?), and was ready for a change.
Also, I'm not sure a lot of people know this, but Warrior players worked hard to convince Durant to come to GS. This wasn't a "we lost the Finals, we could use you" type situation.
They recruited him all year. Green, especially.