Reason 1: Defensive FG%  
The Celtics are leading the league in defensive field goal percentage, at 38.9%.  This is, in my opinion, the single most important defensive statistic.  If you hold the other team to a bad shooting percentage game in and game out, that means a couple of very good things are going on: you obviously have good individual defenders, and no one is getting out of position.  Staying in position and bothering every shot is much better in the long run than, for instance, going for a lot of steals, or worse, "deflections," as Rick Pitino liked to talk about.  You might get a few steals doing that, but when you don't come up with a turnover, chances are you'll be out of position watching one of the bad guys all alone under the basket dunking the ball.  Same with double-teaming and trapping the ball too much.

The defensive statistic that gets talked about most often is points per game, but this one can be deceptive, too.  Want a low defensive PPG?  No problem, hire one of the "defensive genius" coaches who seemed to be running nearly the entire NBA in the 90's--you know the ones: those guys who liked to call every play for the offense, making their point guards stand motionless, dribbling the ball in place until there were eight seconds left on the shot clock, then going into a desperation play.  This does have the effect of keeping the ball away from the other team, like running the ball when you're playing the Indianapolis Colts, but if you don't score, all that means is that you end up losing 85-80, instead of 120-115.  You look like a defensive genius, but you still lose.  The Celtics do not do that--they are moving the ball and scoring, but, just for good measure, they happen to be leading the league in this category as well at 87.14.

 Anyway, leading the league in defensive FG% means you're making the opposition take bad shots all game long, which is the surest way to keep them from outscoring you. 

Reason 2: Assists
The Celtics are currently in fourth place in the NBA in total assists with 24 APG (Utah is in first place with 27.8).  By itself this would be very good, but what makes it even better is who is getting the assists--everybody.  The Celtics' assist leader is Kevin Garnett, which is no great surprise, since he sets up on the high post and gets the ball early and often--but the great part is this: he's averaging 4.9 assists per game.  There are 31 players averaging more assists than Garnett.  This fact, along with their fourth place team standing in APG shows that everyone on the team is involved in moving the ball and finding the open man for high percentage shots. 

 Reason 3: Offensive FG%
 In addition to leading the league in defensive FG%, the Celtics are also leading the league in offensive FG%.  This does not just mean that the Celtics have players who can shoot; every team has players who can shoot.  The most important thing this statistic shows is that each player is getting the ball in a position where he can shoot.  Kendrick Perkins is not going to be mistaken for a great shooter any time soon, but he is shooting 60.7% because he's getting the ball under the basket where he can score.  Rajon Rondo was supposed to be a huge liability shooting the ball, remember?  He's shooting 52.6%.   

 So, there they are, the big three reasons the Celtics have turned into one of the top teams in the NBA in the early going.  Seven good games to start the season is not enough for Celtics fans to cancel their June travel plans over, and they may very well come back to Earth a bit, but these numbers are not accidental.  The Celtics are playing smart, effective team basketball, and as long as they stick to it, they will be very hard to beat.