There is a story about the Christian Priest called Valentine who was martyred by Emperor Claudius sometime around 270CE for various crimes, including being Christian and healing his jailer's daughter from blindness. When he wrote to her he signed the letters “from your Valentine”.
Then there is the story about a Bishop, St Valentine of Terni, who defied the Emperor's orders and secretly married couples so that their husbands didn't have to go to war. Because of this, his feast day (because all saints have them) is dedicated to love.
There are probably more Valentines out there, these were the first two that I found.
However, if you dig a little deeper, you will find that there is actually a very pagan origin to this semi-christian day. Isn't there always?
When you look back to pre-Roman times you find smack dab in the middle of February a festival called Lupercalia. Unlike sweet and romantic Valentine's Day, Lupercalia was centered around animal sacrifice, sex and fertility rituals.
No one knows the exact origin of Lupercalia, but it has been traced back as far as the 6th century B.C. And is believed to be held in honor of the she wolf who mothered Romulus and Remus, twin founders of Rome. And to please the Roman fertility god, Lupercus.
The festival was believed to begin with a sacrifice of goats to Lupercus. After that came the feasting, and then the fun began. The priests, known as Luperci, would take strips of hide cut from the goats and run naked around Palantine whipping any woman who came close to them. There is some debate, but its believed that being lashed by these thongs could help you conceive and have a easy, safe birth.
After that, men would randomly choose a woman's name from a jar, this was their partner for the duration of the festival. Sometimes these couples would opt to stay together until the next year's festival, sometimes they decided to simply get married and stay together.
As the Christian faith grew they took these older pagan rites and sanitized and Christianized them, giving us the innocent-by-comparison Valentines Day, focused on love, romance and chocolate!