Well, it's that one day a year when we all express our love for others even if we don't really feel it. But hey, we're helping the economy, right?
As expressed in this very funny video, saying "I love you" probably means less on Valentine's Day than on any other day of the year--ironic, isn't it? I think the guy in the video sorta messes up the doctor metaphor though. It's more like going into a doctor's office and being really touched that the doctor thought to ask you how you were feeling. That's what he's supposed to do. If he calls you a week later just to check on you, then you know you have a good doctor. Likewise, if your Valentine is still sending you cards and gifts this time next month, then you should really feel special.
And if not, try some of these.
(Hat tip:kottke and Tulip respectively.)