Where did it all start?
Originally, the tradition of the day goes back to one or more Christian martyrs named Valentinus, who according to tradition have suffered martyrdom by decapitation.
The day of remembrance was introduced 469 by Pope Gelasius I. for the whole church, but in 1969 it was removed from the General Roman Calender of Saints for the following reason: "Though the memorial of Saint Valentine is ancient, … apart from his name, nothing is known of Saint Valentine except that he was buried on the Via Flaminia on February 14."
There are also suggestions that Saint Valentine performed clandestine Christian weddings for soldiers who were forbidden to marry. The Roman Emperor Claudius II supposedly forbade this in order to grow his army, believing that married men did not make for good soldiers. However, George Monger writes that this marriage ban was never issued and that Claudius II told his soldiers to take two or three women for themselves after his victory over the Goths.

Saint Valentine of Terni and his disciples. Codex: Français 185, Fol. 210. Vies de saints, France, Paris, 14th century Source: Public Domain

St Valentine baptizing St Lucilla, Jacopo Bassano Source: Public Domain Wiki Commons
According to legend, in order "to remind these men of their vows and God’s love, Saint Valentine is said to have cut hearts from parchment", giving them to these soldiers and persecuted Christians, a possible origin of the widespread use of hearts on St. Valentine's Day.
Saint Valentine supposedly wore a purple amethyst ring, customarily worn on the hands of Christian bishops with an image of Cupid engraved in it, a recognizable symbol associated with love that was legal under the Roman Empire; Roman soldiers would recognize the ring and ask him to perform marriage for them. Probably due to the association with Saint Valentine, amethyst has become the birthstone of February, which is thought to attract love.
In the Anglo-Saxon language area, this commemoration day goes back to a poem by the English writer Geoffrey Chaucer, called "Parliament of the Birds", which was first performed in 1383 on the occasion of a Valentine's Day celebration at the court of King Richard II. This poem depicts the birds gathering around the "Goddess Nature" on the same day of the holiday so that everyone can find a partner.
In the late Middle Ages, the Valentine's Day was also considered a “ Lostag” and as such was also called "Lover’s Day". “Lostage” are actually the religious holidays, on the basis of which one could make, according to ancient folk beliefs predictions about the weather conditions of the following weeks or seasons.
However, it seems to have something to do with lottery too. Since the 15th century Valentine couples have been formed in England, who send small gifts or poems. The couples formed by lots on the eve or by the first contact on Valentine's Day.
The modern cliché Valentine's Day poem can be found in the collection of English nursery rhymes Gammer Gurton's Garland (1784):
"The rose is red, the violet's blue,
The honey's sweet, and so are you.
Thou art my love and I am thine;
I drew thee to my Valentine:
The lot was cast and then I drew,
And Fortune said it shou'd be you."
In the Victorian era, the custom among lovers was to send each other decorated cards.
English emigrants took the Valentine's Day tradition to the United States, and thus it came through US soldiers after the Second World War to the western part of Germany. In 1950 they organized the first "Valentine's Ball" in Nuremberg. Valentine's Day became generally known through the increasing use of florists and the confectionery industry before February 14th. Even in Germany, flowers are still the most common gifts on Valentine's Day. For Valentine's Day 2013 alone, Lufthansa Cargo transported 1,000 tons of roses to Germany.

Scan of a Valentine greeting card dated 1909 Source: Public Domain, Wiki Commons