It's not even that, so much as the fact that it suddenly becomes perfectly okay - compulsory, even - to shove your relationship in everyone else's face all day, and just generally ignore everyone you're not dating.
As far as for people in relationships, I don't understand why Valentine's day, anniversaries, and birthdays are so important anyway. "Hey, I love you not because you mean something special to me, but because the calendar says I should." How romantic.