If Harrison's demeanor and hard hits remind you of Joe Greene, there's probably a good reason. In 1974 before any Super Bowls, but already holding an 8-2 record, the Steelers had their butts kicked by the Houston Oilers. Joe Greene was disgusted by the loss, and grew infuriated after watching the Dolphins man-handle the Bengals the next night on Monday night football. So upset in fact that he almost quit the team the next day. Quoting "Mean" Joe: "That's what I want us to be like. I came into our team meeting on Tuesday, and I really didn't like what I was hearing, so I went and cleaned out my locker and walked to my car."
Is it a valid comparison to Harrison's threat to quit? I think so. Both of them only know one way to play the game: ferociously. Both are completely unable or unwilling to compromise on that, whether compromising means playing on a team that Greene perceives as not having that same drive (as false as that proved to be), or playing for a league that won't allow Harrison to play the game at the speed and intensity that he's capable of (as false as that will probably prove to be).