IMNSHO (and probably an unpopular one at that), I'm going to go out on a limb and say that being Kermit might have gone to Steve's head, and sometimes, a supercharged ego can cause a person to become Dr. Jekyll and Mr. Hyde, which would explain why he might have been difficult to work with (I know that's been said, and I'm sorry for repeating myself, but my brain is on overload). I also agree that it's the material that makes Kermit come off as bitter, angry, and depressed. Though Cheryl should watch some episodes of the Muppet Show, as Jim's Kermit wasn't all sweetness and light. He's had several angry moments (Loretta Swit's episode, anyone?)