I hate to say it's all about the refs, but it's really all about the refs.
IF the Celtics are allowed to hack, push, shove, and manhandle the Lakers in the paint they will win most games. They weren't allowed to do this in Games 1, 2, or 3 and it affected their play severely. In Boston, after Doc's tirade, they were given a lot more la ude and the end result was something that looked eerily similar to 2008.
Don't expect the Celtics to get the same kind of ref treatment in LA. It won't happen, if the refs even try to do that there will literally be rioting in the arena and massive chaos. The Celtics will have to play solid defense WITHOUT fouling, their role players must come up big, AND they need their Big 3 to play good games.
I'm confident LA will win Game 6. It's Game 7 that worries me.