St. Valentine did exist - several historical Valentines are known, in fact - and in particular, the hero of our story lived during the third century, A.D. The story has it that St. Valentine was queried by Roman emperor Claudius as to whether he believed in God. He answered that he did, and Claudius tried to persuade him to convert to paganism and comply with the order to not perform any marriages.
Whether our practice of sending Valentines to one another truly was born of this depends on what you want to believe. And surely Hallmark had a hand in it all, but it still is an endearing holiday, whether you have a true sweetheart or not. Do we not all have friends? Special family members? They can be our Valentines, too!
Happy Valentine's Day!