And he has been criticized as well by many basketball analyst, leaving Nets and then the Warriors winning the championship? That could really hurt Durant inside that's why he wanted to leave and find a new team to win again. But time is running out, it will be hard to find a championship caliber team in exchange of him.
Actually Durant should not be surprise that the Warriors won a championship because the Warriors are already a championship team before he joined the team. He only made the Warriors better but they are still good even without him, as Draymond said, "we don't need you", he was right.
https://nba.nbcsports.com/2018/11/16/draymond-green-reportedly-told-kevin-durant-we-dont-need-you-we-won-without-you-leave/