Durant doesn't have the 'dog' in him to get this over the line at the Thunder, so he bails. Same as Lebron did. It made LBJ a better and more confident player once he won the rings, so he went back to test himself at Cavs.
Right now, this move by KD wreaks of "I can't be bothered working hard, so I'll take the easy route". He might try and go back when he's won a couple.
In saying that, what was the quote from GSW the other day "If you stay with Thunder, you may win a ring or two and we might win a ring or two. But if you come here, we will both win a whole bunch of rings". So pressure is on them to deliver, which proved to be too much for Miami in their first year.