There is a famous quote that says the only difference between a tragedy and a comedy is the ending. This means that if a story ends with a marriage, celebration, or other happy finale, the rest of the work takes on the tone of that joyous occasion, making it a comedy or a romance. If, however, the ending is sad or tragic in some way, the remainder of the work becomes tragic simply knowing that it ends in heartbreak.
This poem by Chaucer sets up a love story between Troilus, a Greek soldier, and Criseyde, a Trojan refugee. During the war, the two become infatuated with one another, and as they grow closer, they get separated, with Criseyde being returned to the city in exchange for a prisoner. She promises to deceive her father and escape, returning to Troilus in ten days. However, she begins to realize this is unlikely, and the two are separated forever as she accepts another man’s proposal. Troilus realizes she’s not returning and is killed in the battle. This is a very tragic ending, as the lovers are never reunited, and Troilus dies a death made more tragic by the fact that his love has left him.
Chaucer's medieval rendition of Troilus and Criseyde can be seen as a romantic tragedy because the story chronicles the fateful (and ultimately tragic) relationship between the two lovers. In an ironic twist, Troilus, a young Trojan warrior, denounces love, yet after the God of Love shoots him, he falls in love with Criseyde, a beautiful Greek widow. And, although Criseyde mirrors Troilus's affection, she betrays him by choosing a Greek man, Diomede. Troilus is naturally brokenhearted, and in another ironic twist, he dies while fighting the Greeks. After his death, Troilus is released to the eighth sphere where he contemplates the pointlessness of love and war.
Based purely on its plot, Chaucer's version appears as tragic. However, what contributes to the story's tragic nature even more so is the medieval idea of fortune as a marker of pre-destined events. Put plainly, Troilus was forced to learn a difficult lesson on account of something he had no control over -- Criseyde's untimely betrayal. His fate (or fortune), in a sense, was already determined. And while he died tragically, he was able to reflect upon the futility of love and war after death.