Why is tragedy often called the most "optimistic" of the four genres?
The tragedy is most often called the most optimistic of the four different genres of dramas (tragedy, comedy, melodrama, and farce). To understand what make the tragedy optimistic, one must understand what optimistic means. Optimistic means to have hopefulness and confidence regarding the future.
That being said, one also needs to understand the differences between the four main types of dramas.
Tragedy: The tragedy depicts the downfall of a great person. While the definition has changed overtime (today a tragedy simply deals with subject matter of a very serious subject matter and death), the fact that death is involved makes this genre more serious than the others. While one would think that a tragedy could not bring out a feeling of optimism, it can.
Comedy: The comedy is light in tone and meant to allow the audience comic relief from life in general. These plays, typically, have happy endings.
Melodrama: The melodramatic play is one which is one-dimensional and simplistic. It offers the story of a hero, a heroine, and a villain. This story follows a typical plot: the hero must rescue the damsel in distress from her captive, the villain.
Farce: The farce is considered a comedy. This genre over-exaggerates characters and situations. The action is one-dimensional and the characters are stereotypical.
Therefore, based upon these definitions, the tragedy is the only genre which forces watchers, or readers, of the play to examine the character and their actions. Something is to be learned by what happens in the tragedy and the audiences is expected to leave questioning their own lives and decisions. It is through this questioning that one can find optimistic views on life. This is done by forcing the reader/watcher of the play to examine how they are living their life and what they need to do to insure their own survival in life.