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.