I think it's a combination of things:
-Fatigue
-Lack of Leadership
-Lack of Motive (they're going to make the playoffs)
-Crappy rosters as a result of injures.
I think you hit the nail on the head that KG is the leader. While I'd say Pierce and others have leadership quality, KG is the most vocal and not afraid to get in people's faces. THAT is what this team is missing, I just wish he'd get in Rasheed's face a bit too.
But last night they had a roster of Davis, Scalabrine, Tony Allen, Ray Allen, and Eddie House on the court at the same time. I'm sorry, but the Celtics can't beat the Mavs if that's who is on the court. Most teams would outmatch that odd group of 5.
Why win now? Home court? Sure it's important, but are 40 games played "all-out" worth 1 extra game at home? I'm not condoning it, but you can kind of see that mentality with this team.
I'd like for the Celtics to get healthy and flex their muscle for a game or two just to show us what we've been missing.