Set in France and Louisiana in the early 18th century, the story follows the hero, the Chevalier des Grieux, and his lover, Manon Lescaut. Des Grieux comes from noble and landed family, but forfeits his hereditary wealth and incurs the disappointment of his father by running away with Manon. In Paris, the young lovers enjoy a blissful cohabitation, while Des Grieux struggles to satisfy Manon's taste for luxury. He scrounges together money by borrowing from his unwaveringly loyal friend Tiberge and by cheating gamblers. On several occasions, Des Grieux's wealth evaporates (by theft, in a house fire, etc.), prompting Manon to leave him for a richer man because she cannot stand the thought of living in penury.
The two lovers finally end up in New Orleans, to which Manon has been deported as a prostitute, where they pretend to be married and live in idyllic peace for a while. But when Des Grieux reveals their unmarried state to the Governor and asks to be wed with Manon, the Governor's nephew sets his sights on winning Manon's hand. In despair, Des Grieux challenges the Governor's nephew to a duel and knocks him unconscious. Thinking he had killed the man and fearing retribution, the couple flee New Orleans and venture into the wilderness of Louisiana, hoping to reach an English settlement. Manon dies of exposure and exhaustion the following morning and, after burying his beloved, Des Grieux is eventually taken back to France by Tiberge.

Question: Why does Manon Leave Des Grieux?
Answer: Because Des Grieux becomes poor after unfortunate events that cost his money. 
Alternative answer: the fear of living in penury