According to Aristotle, the main component of a tragedy lies in action, or the imitation of an action.For each character, it is by his actions that he is made happy or sad.He further notes that in tragedy, character is secondary to action.He also notes that the characters, through their actions, should invoke pity and fear from the audience.To me, this seems like a hopelessly narrow and confining way of looking at tragedy.I get the feeling that perhaps Shakespeare too felt confined by these conventions.That would at least explain the presence of his'problem plays' that do not easily fit into any one category.
I think most of these genre conventions stem from people's desire for fulfilled expectations.If Romeo and Juliet had been announced as Shakespeare's new comedy, I imagine there would have been some confusion and even anger among the audience when they saw that it was clearly a tragedy.I guess the bottom line is, people like a good spectacle, and they like to have their expectations fulfilled.Hence, the necessity for different genres.They allow you to know what you're getting into beforehand.I say, to hell with genre!Let the artist be an artist! Do away with the idea that comedies must contain elements A, B, and C in order to be valid.I find Aristotle's description of tragedies so specific as to be narrowing, and therefore exclusive.