Theres actually a rather interesting myth about how Helen of Greece became Helen of Troy, the myth goes like this:

When the Hero Peleus was married to the sea-nymph Thetis, there was a big banquet on Mount Olympus which all the Gods and Goddesses were invited to, save one. This jealous goddess decided to get revenge by sowing discord among the Olympians, so to do this she sent a Golden Apple to the banquet upon which was inscribed "To the Fairest." All the goddesses of course immediately stepped up to claim it causing a dispute of who was the most beautiful. The goddesses asked Zeus to judge who politely declined (wise man) than decided to have Paris of Troy who was quite the ladies man decide. So they all went down to Troy in their best outfits and offered Paris secret bribes should he choose them, Aphrodite however offered Paris the most beautiful woman in the world. Paris chose Aphrodite and Aphrodite made Helen who was Menalaus' wife in Sparta fall in love with Paris, so Paris went to Sparta and fetched her and they went back to Troy together, thus starting the legendary Trojan War.