Shrek was cursed by an evil witch...
The curse forced him to be unable to speak without singing.
Unsure of what to do, Shrek visited Juan the Wizard in the neighboring swamp. Juan told Shrek he'd need to make a potion from toadstools, eye of newt, and the bones of the freshly deceased.
Shrek said he could handle the toadstools and eye of newt but he refused to kill an innocent person to solve his problem.
Juan understood and said that for a modest fee he would break into the nearby morgue and steal one for him. Shrek agreed.
The following day Juan the Wizard delivered as promised. After he left, Shrek began to prepare the potion in a large cauldron. Just as he was about to add the cadaver, Donkey burst through the door.
Mortified, he screamed, "Shrek! What the hell is that?"
Shrek turned and sang, "Some body Juan stole me."