The analogy was made before this series that the Lakers were the Unstoppable Force on offense (and they looked like it, before the Celtics got to them), and the Celtics were the Immovable Object with their tight defense. So how did it work out, and why?
Initially, it was a tough balance. Could have gone either way, and did. The teams exchanged wins and losses, and seemed very closely matched. But time helps the Immovable Object far more than it does the Unstoppable Force.
Removing metaphors now...
A highly tuned defense gets better and better as it gains experience with a particular offense. But the OPPOSITE is true for offenses... even great ones.
A great defense GROWS, ADJUSTS and CONFORMS to handle the challenges it meets. The more time it has to learn what the opponent's offense is doing, their strengths and weaknesses -- the better a good defense gets. (Especially when you have a defensive savant as the assistant head coach.)
You can see specific examples of that, microcosmically, in the way Kobe and Gasol have been defended. No longer are they able to run riot over the Cs. Kobe has to put his superman cape on to score now, and Gasol is being shut down altogether.
But a great offense DOES NOT TYPICALLY GET GREATER when faced with great defense. It stays about the same (barring extreme genius working that side). By staying about the same, the offense actually loses ground over time, because it's facing a continually improving defense. And this is what we've seen. As the Lakers' offense has stayed pretty much the same, the Celtics' defense improved with every game. Now we've reached the point where the Cs have the Lakers' number. Again. Just like in 2008.
I believe we may yet see a blowout to end this series. If, that is, the Celtics can get all their offensive guns to fire reasonably well in the same game. Because while our offense fires away, our defense has already taken control of LA's offense.
The core of the Celtics in our time has always been defense. They've said it a million times. And you know that old adage that claims 'Defense Wins Championships'? We're about to see that truth play out one more time.