It's just the way Brock Lesnar is.... He's a very aggressive, mean, in-your-face kinda guy. Mir made him look like an idiot (tapping out is the worst way to go, for me...) so he wanted his revenge. When you couple that with the fact that hardly any fans of UFC give Lesnar any credit (oh, he's just a WWE fake, he can't fight) blah blah blah, it's gonna start up a fire.
He's built his whole career on being a villain. He's not a nice, happy go lucky fighter... He's an *******. Either you love him or you hate him for it... Personally, I love him for it. The UFC needs some personality. I'm not saying go full on WWE with scripts and predetermined outcomes, but who doesn't like a little drama when they see two men go at it... It takes both competitors to a higher level of intensity.