I never understand why Valentine day has been associated with love while act of saint Valentine was courageous & charismatic (once in his life time). To proof my point i search in historical document and found following points
Courage of Saint valentine: “Saint Valentine was imprisoned for performing weddings for soldiers who were forbidden to marry and for ministering to Christians”. To go against the Rome emperor needs courage. what happen to those soldiers after marriage is unknown perhaps they have been punished or may be absconded from Rome but Priest paid the price . Note its not about Helping Couples who were in love but to help people to start there families ( Love don’t need marriage to be proved).
Charisma of Saint valentine: “He is said to have healed the daughter of his jailer Asterius when he was imprisoned”. Legend states that before his execution he wrote "from your Valentine" ( this is the form of expression every one write it with in letters) as a farewell to her. I don't see any romantic love between these two as there ages are not known and a healer always get lot of respect from the persons.
Heart symbol for Valentine: I become curious how the Heart comes in picture and voila! found the answer as Saint Valentine is said to have cut hearts from parchment, giving them to the soldiers ( note not to the girl or couple it’s soldiers may be as symbol of his blessings)
What about 14th Feb: Saint Valentine was buried on the Via Flaminia on February 14. In Ancient Rome, Lupercalia, observed February 13–15, was an archaic rite connected to fertility. Lupercalia was a festival local to the city of Rome. The more general Festival of Juno Februa, meaning "Juno the purifier "or "the chaste Juno", was celebrated on February 13–14.
Note: The celebration of Saint Valentine did not have any romantic connotations until Chaucer's poetry about "Valentines" in the 14th century.
Valentine's letters: In Middle Ages, men drawing the names of girls at random to couple with them and around 1600, this has been replaced with a religious custom of girls drawing the names of apostles from the altar.
So why Valentines day Became so famous: That’s very difficult to answers but seems it has been famous by the Poet Chaucer's by following poem “Parlement of Foules” (it has almost 700 lines)
For this was on seynt Volantynys day
Whan euery bryd comyth there to chese his make.
["For this was on Saint Valentine's Day, when every bird cometh there to choose his mate."]
But here is the catch This poem was written to honor the first anniversary of the engagement of King Richard II of England to Anne of Bohemia.A treaty providing for a marriage was signed on May 2, 1381 and mid-February is an unlikely time for birds to be mating in England.
So what one should do: Ok i gave up now history is very confusing and we celebrate many ritual for wrong reasons also. We need special day for every thing due to social,Economy & marketing reasons and if some Famous/powerful persons declare a day special people start believing in. so we have a day dedicated for Love if you believe in it Celebrate it if you don't then enjoy the life & dedicate any other day ( may be every day) for your dear and love once . – Enjoy
Hayle Bishop Valentine whose day this is
All the Ayre is thy Diocese
And all the chirping Queristers
And other birds ar thy parishioners
Thou marryest every yeare
The Lyrick Lark, and the graue whispering Doue,
The Sparrow that neglects his life for loue,
The houshold bird with the redd stomacher
Thou makst the Blackbird speede as soone,
As doth the Goldfinch, or the Halcyon
The Husband Cock lookes out and soone is spedd
And meets his wife, which brings her feather-bed.
This day more cheerfully than ever shine
This day which might inflame thy selfe old Valentine.
—John Donne, Epithalamion Vpon Frederick Count Palatine and the Lady Elizabeth marryed on St. Valentines day
Reference : Various sources on internet specially Wikipedia