Thinking about the guilt of uneaten sushi, I think it must stem from two facts. One, the guy who made it for you, with all the craft and artistry in his arsenal, is
right there. You feel like you're personally insulting him if you don't eat every piece. (And--this may be wholly imaginary, or may not be--it feels like a cultural issue is at stake, in that not finishing everything might be a terrible breach of Japanese etiquette, and understood to be a sign of disrespect.) Two, the rawness of the fish brings to mind more than most foods that life died for your enjoyment, and how dare you disregard that sacrifice.
Just a theory.
Pithy quote here.