It is a domanda we've all been wondering for years. Here are the superiore, in alto 10 possible explanations I was thinking of one giorno when I was bored:

10.) He has terminal brain cancer.
9.) He's not crazy, it's that Moose-Haired Little Nuisance's fault!
8.) He's inhaled a too much ammonia.
7.) He was born crazy. That's why the best job he could get was as a hospital janitor.
6.) His hard-ass dad abused him as a little kid.
5.) He was dropped on his head as a small child da his equally insane parents.
4.) He knocked himself in the head with a knife-wrench.
3.) It's all the alchohol.
2.) He's not insane. It's just fun to pretend.
1.) It's all an act to impress the equally-crazy Blonde Doctor.