Most people trace the story of Saint Valentine back to a Roman priest in the year AD 270. He was arrested and imprisoned for performing marriage ceremonies for Christian couples at a time when such ceremonies were prohibited (as married men were exempt from the Roman army). Valentine also may have aided other Christians who were being persecuted during the reign of Emperor Claudius Gothicus (Claudius II).
Valentine was brought before the emperor and told to renounce his faith, but even under extreme torture he refused to do so. According to legend, couples whom he had married brought him flowers and gifts while he was in prison, which gave rise to the tradition of giving flowers and gifts in his honor. (www.insidecatholic.com)