I think it has something to do with opera. No happy ending and all that.
From _What’s Opera, Doc?:_
Elmer: “Bwoon Hilda, you’re so wuv-wee...”
Bugs: “Yes, I know it. I can’t help it...”
[Later...]
Elmer: “Wut have I done??? I’ve killed da wabbit! Poor wittle bunny! Poor wittle wabbit!”
Bugs: “*Well, what’d you expect in an opera? A _happy_ ending???*”