Back in the 14th century, the French were under attack....again. Of course they were losing, because that's all the French can do successfully (besides being rude, of course). Back in the olden days, there were castles and towers and stuff, and one that was under attack held a woman by the name of Sylvia Palentine. Ms. Palentine had a boyfriend who was stupid, and despite his best efforts, he hid his face under his bloomers and ran off when the going got tough, because, as established, he was French. The castle was destroyed with Palentine in it. When her idiotic boyfriend returned, he was so traumatized that when he explained the tragedy to someone, he called her Valentine instead of Palentine. Thus, the day was born.
What about this cupid fellow? Well, the battle of Valentine was mostly done by crossbow and regular bow, as modern-day conveniences were not around, like tanks and H-bombs. So when the story of the Valentine's slaughter was abounding, the word was that arrows were being used, and just like the "telephone game" the generations soon surmised that Valentine was being attacked through passionate love, and the thing that won her over was a near naked midget shooting a "love arrow" at her. I personally blame the hippies, because that sounds like a hippie explanation.
Al Capone was another famous "Valentinsian" hero as he took part in the famous "Valentine's Day Massacre". There were two gangs trying to win the affections of some lady named "Chicago". Chicago was torn between these two groups and cupid couldn't help because as established, cupid was invented by the hippies and this was in the 1920's, when prohibition was awesome. Al Capone said "See here, Chicago is my turf (dirty!) and this here money is mine!". Shots rang out and Al Capone won. Since the blood that was everywhere was red, the new official color of Valentine's Day is red, so this event is significant for the development of the modern day holiday.
Another aspect is that there is an opposing movement called "Antivalentinism", but this can only be five paragraphs long, so maybe I will elaborate on it in the future. As you can see, the history of Valentine's day is brutal and bloody, and we have changed it into a day of sugar highs and an increase in the chances of us having sex. This totally makes sense. So whenever you look into those limped pools of awe in the eyes of your girlfriend this day, remember, behind every heart-shaped box of chocolates, there is a woman being murdered, a near-naked midget shooting deadly arrows, and a gang fight. If that ruins the mood, then forget the holiday and enjoy the time you have together anyhow, as Valentine's day is evil.
History of V-Day