Why is Twelfth Night A Comedy?

It's a comedy because the Countess Olivia falls in love with a girl who is pretending to be a boy named Cesario, however, 'Cesario' or real name Viola, has a twin brother called Sebastian (They were separated at a ship wreck, and if you are wondering why shes pretending to be a boy its because she needs work to earn money) and Olivia bumps into Sebastian thinking its 'Cesario' and she repeats her love for 'Cesario' to Sebastian. Olivia asks Sebastian to marry her, Sebastian however is completely oblivious to whats happening but seen as Olivia is beautiful he agrees. In the end all the confusion of identity works out, and Olivia keeps her vows to Sebastian and 'Cesario' comes clean about being a girl and the Duke who she is deeply in love with finally realises that he loves her.

Hope it helps!