Reason I say that is because during all of the pre-fight smack talk between himself and Mir, Brock seemed genuinely pissed off. And Mir kept antagonzing. To be honest I think Mir was just trying to hype the fight like many fighters do, understanding that it will increase interest and PPV buys, while Lesnar, still new to the sport was taking it all personally.
You could say, well Lesnar just played his heel character and was seeing it all the way through. Well, if I reflect on the Couture fight it says otherwise. Couture had little negative things to say about Brock, so Brock didn't sya to much to Randy. They just got in there and duked it out, far cry from the build up of Lesnar/Mir.