From the rumors, he wanted to get away from Lebron, all the "circus" that surrounds him and the extra Circus of this season. He just wants to play basketball.
Playing with Hayward,Horford,Brown,Smart, they're straight up no drama ballers. Tatum could even be considered that from interview with his mom, just seems down to earth.
And you have a coach and organization that are the same. Where most organizations are worried how to fill seats, Boston first worry is titles.
This could be a good fit.