This from The History Channel:
According to one legend, Valentine actually sent the first ‘valentine’ greeting himself. While in prison, it is believed that Valentine fell in love with a young girl — who may have been his jailor’s daughter — who visited him during his confinement. Before his death, it is alleged that he wrote her a letter, which he signed ‘From your Valentine,’ an expression that is still in use today. Although the truth behind the Valentine legends is murky, the stories certainly emphasize his appeal as a sympathetic, heroic, and, most importantly, romantic figure. It’s no surprise that by the Middle Ages, Valentine was one of the most popular saints in England and France.