1) He won't be trade fodder, which he will be if he resigns with us (and I prefer it that way.)
2) He is guaranteed to be on a Top 3 team in the East, with a perennial shot at the NBA Finals.
3) He gets to play with one of the best passers in the game in Lebron.
4) He gets to play with two guys who are constantly double-teamed (Lebron, Wade), which will lead to many open 3s.
5) No Rondo.
6) No Danny Ainge.
7) No snow or cold weather.

No state taxes.
9) And if he really hates Boston because of the trade offers, he gets to play for the Celtics' most-hated in-conference rival at the moment, and he gets to prevent them from going to the Finals (I don't believe #9 applies, but hey you never know.)