In my opinion, there are several reasons why we won't win 12 in a row:
1) The guys that have been healthy simply haven't been playing well enough lately.
2) Usually there's an adjustment period when such a marquee player returns to the lineup, never mind when he's coming back to a different team than he left (additions of Steph and Mikki).
3) It will be tough to beat ATL twice, Orlando, and Cleveland, with 3 of those 4 games being played on the road.
4) No matter the sport, the league, or the opponents, it is very tough to win 12 games in a row.
5) If the first 4 reasons aren't good enough for you, keep in mind that the Celtics will be more concerned with entering the playoffs rested and healthy than winning games.