Paris's bull and Ares as a bull went against each other, Paris lost, but kept his word, and Zeus saw that, and was impressed by it.
Zeus planned a wedding for Thetis and Peleus, any invited every God and Goddess , but one, her name was Eris, the goddess of discord, but she found out there.
Eris threw down an apple that symbolized, whoever had the apple was the fairest of them all, and all goddess ran to get to get the apple and the only three goddess who made it were Aphrodite, Athena, and Hera.
All three goddess wanted Zeus to choose who got the apple, but he couldn't choose so he thought of Paris, because he is a man for being fair.
Paris was there to judge, and the goddess started bribes, Athena would make him the finest warrior that ever live, Hera would have made him the King of Europe and Asia, and then Aphrodite gave him a a offer he couldn't accept , to marry Helen the most beautiful mortal women in the world, Sparta's wife. Paris accepted the offer.
So Aphrodite went through with her word and so Helen fell in love with Paris and she and Paris ran away together, but Sparta found out and told his men to find Helen, and they found out where she was and then the Trojan War broke out.