He doesn't care that you once shared a cubicle.

"Friends from work?" Pffftt -- that concept has loooong outlived its usefulness. Here on the planet Sakaar, Hulk has the freedom to be Hulk. What does that mean, exactly?  

That means a glorious amount of smashing in an arena filled with screaming fans excited by said smashing -- be it by dark matter hammer, four-hundred-ton mace, or boulder-sized Hulk hands.