Valentine's Day is not a holiday of love, but one of commerce. There was no holiday yet which forces people to buy presents during that time of the year, so they made up one. Easy.
The day I have a girlfriend, she won't get anything from me on that particular day, it would only sound like "here is a present because anyone else buys one as well". She will get a nice surprise on other days, when she is not expecting it and when it comes straight out of the heart.
Valentine's Day, with all due respect, is a commercial fest really.