In truth, the exact origins and identity of St. Valentine are unclear (website)

On February 14 around the year 278 A.D., Valentine, a holy priest in Rome in the days of Emperor Claudius II, was beheaded...

Claudius the Cruel believed that Roman men were unwilling to join the army because of their strong attachment to their wives and families… so Claudius banned all marriages and engagements in Rome. Valentine defied Claudius and continued to perform marriages for young lovers in secret.

When Valentine's actions were discovered, Claudius ordered that he be put to death. Valentine was arrested and dragged before the Prefect of Rome, who condemned him to be beaten to death with clubs and to have his head cut off. The sentence was carried out on February 14, on or about the year 270...

Legend also has it that while in jail, St. Valentine left a farewell note for the jailer's daughter, who had become his friend, and signed it "From Your Valentine."

For his great service, Valentine was named a saint after his death.

Legends vary on how the martyr's name became connected with romance. The date of his death may have become mingled with the Feast of Lupercalia, a pagan festival of love. On these occasions, the names of young women were placed in a box, from which they were drawn by the men as chance directed. In 496 AD, Pope Gelasius decided to put an end to the Feast of Lupercalia, and he declared that February 14 be celebrated as St Valentine's Day.

Gradually, February 14 became a date for exchanging love messages, poems and simple gifts such as flowers.

(http://www.history.com/this-day-in-history/st-valentine-beheaded)