The decision for KG seemed to be between retirement or playing with the Celtics. I doubt he even considered playing for another team.
I think what the Celtics did in the draft might have been the final thing that convinced Garnett to stay. The Celtics selected guys that will help this team win right now. Sullinger and Melo not only compliment the Celtics, they compliment Garnett.
He wants to win again. We saw that hunger in the playoffs. And since he's coming back, the Celtics have a good chance of going far in next year's playoffs.