Short answer: who knows?
A longer answer from Maggi Dawn:
There were at least three Valentines, and we don’t know all that much about any of them. Whether the day is attributed to one or all of them, who knows – but the most popular story centres around a priest who performed illegal marriages. The story goes that the Roman Emperor Claudius II imposed a ban on marriages in order to boost his army. Only single men had to enter the army, and too many men were dodging the draft by getting married. Valentinus, though, in an effort to protect the sacramentality of Christian marriage, performed secret marriages, and when he got caught he was sentenced to death. While he awaited execution, he was showered with notes from young couples extolling the virtues of love over war. (Looks like John Lennon didn’t invent the slogan “Make love not war” after all.) These notes, if they ever existed at all, were supposedly the first Valentines. Poor old Valentinus was executed in February 14th, 269, a bloody end for the saint of love.
A second priest named Valentinus also found himself in prison for helping Christians, and according to legend he fell in love with the daughter of his jailer and sent her notes signed “from your Valentine”.
The third St Valentine, the Bishop of Terni, was also martyred, and yet another Valentinus – a second-century Gnostic Roman teacher – although not a Christian himself, argued that sex and marriage were central to Christian teaching.
It was 469 when “Valentine” was given a feast day, in the hopes of replacing February’s pagan feasts of love and fertility with a theme of Christian love and martyrdom. Judging by today’s customs I’d say the scheme wasn’t altogether successful.