Disclaimer: I don't own Monster's Inc.
Mike had always been a prankster, but, watching the effects of one of his biggest tricks, he wondered just how far was too far to go to get someone to smile.
After Boo's door had been shredded, Sulley hadn't been the same. He went through all the motions of everyday life, working, smiling, but Mike could see the sadness hidden behind every smile, he noticed each time his friend flipped to the very last page of his clipboard. Mike made it his mission to make Sulley happy once again.
Mike had pushed it too far. Sulley was now visiting Boo everyday. He would talk to her, play with her. Mike could only watch sadly as his best friend convinced himself that the dummy in the simulation room, decorated crudely with pigtails and a pink dress, was Boo.