It depends on which translation you have, but several translations offer at least two of the three reasons:
1) The city of Carthage. As Virgil explains in Book 1, Juno loved the city "beyond all other lands in the world" (ll17-18). Unfortunately, hse knew that men of "Trojan blood" would someday destroy Carthage.
2) She was still angry with Paris (a Trojan) for having not been chosen as the most beautiful of the three goddesses: the Greek versions of their names are probably more familiar to you: Hera, Aphrodite, and Athena.
3) The cup-bearer. Hebe, Juno's daughter, was once cup-bearer to the gods. Some sources (often Greek) claim that she gave up her position to marry Hercules/Herakles. Whatever the reason for her absence from the post, she was replaced by Ganymede, said to be a youth so beautiful that her husband (Jupiter/Zeus) fell in love with him and placed him in Hebe's position. Other versions say that Ganymede was the illegitimate son of Juno's husband; in any case, Juno fiercely resented the young man.
Virgil, The Aeneid. Translated by Robert Fagles. Penguin Books, 2006.