Why were the greeks and the trojans adversaries

Paris, a Trojan prince, had stolen away with Helen, Queen of Sparta, and and the Greeks went to war against Troy to get Helen back.  Of course, that's the romantic version told by Homer. More likely, Troy was charging exorbitant tolls on Greek ships trading between the Aegean and Black Seas, and the Greeks decided it would be cheaper to wipe out the city than to continue paying them.