I never celebrated Valentines Day when I wasn't single (except one time, when my ex-husband got me injured on February 13, and took me out to make it up to me). I hate the advertising suggesting that you're not really in love if you don't shower her with diamonds (what if she doesn't like diamonds?) or provide him with the wildest sex of the year (why not have the wildest sex of the year because it's Thursday? Or better yet, try to make your sex life as satisfying as possible every day of the year).
I also hate the bitter mopey-ness that comes with the day. I have friends who have never been in a serious relationship moaning about the day, and friends that are in good relationships moaning about the day (like my best friend: how are you supposed to be romantic with a wired-up toddler when you're 6 months pregnant with the second child, and no one is available to babysit because everyone else wants to be with their sweetie, or is busy drinking their loneliness away). It just seems to be a day for everyone to be insecure and to be sure that everyone else is happier than they are.