From a Kayfabe point of view:
Brock Lesnar is a seemingly unstoppable monster. Despite losing to Cena on his return, he destroyed Cena for 99% of that match, and was pissed when HHH didn't allow Brock to own Raw. Lesnar then provoked HHH by attacking Mr. McMahon, his father in law, and his best friend Shawn Michaels. Lesnar even broke HHH's arm. Now lets go ALLLLL the way back unto when Lesnar was on Raw in the Ruthless Aggression era. He never fought HHH for the World Heavyweight championship, instead he went to Smackdown, so the two never fought. Now, they meet at Summerslam, and Lesnar makes HHH tap out. Thus HHH felt humiliated and trains like Rocky for a rematch. They battle it out at Wrestlemania, and finally HHH evens the score. BUuuuuttttt that's it. Even. Lesnar is a monster, he doesn't WANT to be even, he wants to be on top, the monster, the God, and the King of kings. He doesn't like to be equal, thus wanting to beat HHH again.
The way I see it:
Lesnar leaves because no one can stop him(*coughCenaStillBeatYouDumbasscough*) and he felt like he was at the tip of the mountain, not beating the face of the company, but instead beating a out of shape 40 year old man. Lesnar comes back to save Heyman, and HHH wants a rematch. HHH had bought a new shovel, nice, with a chrome plating that had Lesnar's name on it. HHH digs a nice hole for Lesnar at a boring event, barely anybody will remember. HHH is not happy with this, because the hole was to small and LEsnar crawled out, so there's only one logical reason; bury him at another PPV, the same one he lost at his return last year. Not only will he get the main event and spotlight, but he got a crane and dug the biggest fuckin' hole you can dig, it's more wide then Kelly Kelly's vagina, and more deeper then Simba seeing Mufasa die.
Of course it does seem logical form a kayfabe point of view, but I have no interest in it whatsoever, in matter of fact, I'm more interested in hearing Bret Hard talk about it.