KG has long been criticized for being too unselfish, and I think that's where it comes from - he is constantly trying to get his teammates involved so he is often reticent to take the big shot. He'll always look for the open man first. Unfortunately, sometimes his teams need him to take the big shot.
Part of it also comes from being a jump shooter as much as a post player, so he doesn't instinctively think to drive in and force a layup or get a foul. Sometimes he does, but TD ALWAYS does, because he's less of a jump shooter. In that way TD is more of a C/PF than a PF/SF, whereas KG is closer to the kind of PF Dirk plays.
They had that interview with the Celtics' big 3 a couple months ago, and they asked all 3 of them who they wanted to take the big shot at the end of a game. KG and PP both said Ray Allen, and Ray Allen said "whoever's open." I thought that was very telling.
If we asked the same question of our big 3, I wonder who they would say? I suspect they might say Manu, but they might say Tim.