Poor Helen of Troy... minding her own business... looking hot.
Helen was the subject of some serious male chest-beating and interference in her life from some very jealous goddesses.
But let's backtrack: in Olympus, a fight between the petulant goddesses Athena, Hera, and Aphrodite erupts. The three argue about who among them is the "fairest." The goddesses decide to let the mortal Paris make the call. He selects Aphrodite. Aphrodite "rewards" Paris by causing the beautiful Helen to fall madly in love with him.
Agamemnon, Menelaus' brother, vows to get Helen back. Mayhem ensues...
There is also an other account of the causes of the Trojan war which says that Zeus planned it long before the "apple incident" along with Themis (I think Themis, I don't remember exactly but one of Titanesses) in order to alleviate the pain of mother earth because she is burdened with the great number of men peopling it. Thus, the Trojan war was planned in order to decrease the number of men on earth.
But the account given by jamie-wheeler is the "official" cause given in the poem, though I have to say something about the last part of the answer. It is not certain whether Helen was in love with Paris. Already, this is quite obvious in the poem that she sympathises too much with the Greeks. Plus, in different sources on mythology, it is highlighted that Helen is not really in love with Paris.
One last thing, the revenge retaliation is not triggered by Agamemnon but by the oath that Ulysses had made all the suitors of Helen take: that whoever takes her from Menelaus will have to answer to all others. Of course, that's another part of the myth.