Because he has a heart of coal, and is jealous that Donny can sing better than he can.

Plus, he's getting into character for his role as Scrooge in Dayton's live presentation of The Christmas Carol.

I'm envious of Donny's great singing too, but I still want him back; if only to hear those dulcet tones once more and to gaze in awe at that stylish chicken hat, cocked at a rakish angle upon his Duke-ish of Url-ish dome.

Of course, the above is said with tongue firmly in cheek, and with the greatest respect and highest reverence for those mentioned.

Ian