Cena was like Seth Rollins in that he was a company man and he thought that people like The Rock shouldn't call WWE their home. "This is not your home, this is
my home!

" In his mind, if he's out here grinding every day and representing the company as much as he can, don't come back when you need a check balanced.
Rock thought Cena was corny, didn't like what he stood for, and was part of the reason WWE declined in popularity. He also wanted to prove he was the better man and claim that he beat all three franchise players at WrestleMania (Hogan, Austin, Cena). It was his destiny to be the greatest of the great.
That first match had real heat behind it. You could tell they legitimately disliked each other and it made things feel more authentic. Where they fukked up was doing a rematch. Nobody wanted to see that, and it was set up in an inorganic way (Rock gets a title shot at the Royal Rumble six months in advance, wins the title, and ends Punk's reign just so he can drop the title to Cena).