This is a very good question and you would think that the Iliad would address this question at greater length, being that this was the causus belli of the Trojan and Greek conflict. However, if you look at the Iliad, it starts in res media, that is, in the middle of the conflict. To be more precise, we are in the tenth year. In light of this, the story just assumes knowledge of prior events.
The back story is that the gods fated Helen to fall in love with Paris. This point is alluded to the book 24.24-30 in Homer's Iliad. Here is the story. According to mythology, there was a wedding between Thetis and Peleus. However, the goddess Eris was not invited. She was so incensed at this fact that she caused there to be strife among three goddesses. She brought a golden apple, which three goddess claimed - Athena, Hera and Aphrodite. To decide the matter of who would get the apple, they decided to go to Paris, a mortal, to decide a contest, since he was known for fairness.
Paris was to decided who was the most beautiful. The goddesses promised certain things to gain the upper-hand. Hera promised rule over Asia and Europe, Athena promised victory in battle and wisdom, and Aphrodite promised the most beautiful woman in the world. Of course, she failed to mention that this woman was married! Paris picked Aphrodite. From this perspective, Helen fell in love with Paris, because the gods made it so.