This is sort of like the question of celebrating Halloween. It's not that we celebrate, it's really how the world celebrates the holiday.
St. Valentine, while not an official saint, was a real person who died a martyr, for the love of Christ. We celebrate all martyrs. Now, should we go out and buy pajamas, chocolates, roses, or jewelry for our lady-love because it's Valentine's Day? No, not necessarily. The way the world celebrates this day is totally apart from how the Church does, and that's true of any religious holiday-Halloween, Mardi Gras, Christmas Eve, New Years Eve are all usually celebrated with other-than-holy celebrations, but they are the preparation time for holy days. Same with St. Patrick's Day as well as Valentine's Day.