It becomes harder for these people to stop feeling sorry for themselves and focus on other things, when red hearts and bouquets of flowers and locked hands and mashing lips are thrown in their face. It's a bit flippant for someone in a happy relationship to chastise the sadness of those not in one, and for them to pretend that Valentine's Day isn't about being in love. Like the people in love are really going to keep their night out with their sweetie all platonic and chummy. Going out with friends and having fun is better than sulking in a closet all night feeling sorry for yourself, but it won't correct the basic problem, the hole in your heart that widens every February 14th. That's not the point, and that's not what Valentine's Day is really about.
Life is hard enough already.
Maybe with time, it'll happen, but then the artificial timing of a set calendar date inevitably conflicts with people's natural cycles. There is too much expectation created by this arbitrarily set day of love. Finally, of all the holidays during the year, Valentine's Day is the most offensive crime of commercialization on the calendar. Basically, the day has evolved as a capitalist tool to promote consumerism. Commercialization occurs not only through the mandatory flowers, jewelry, and expensive restaurant tabs.