I've never been upset about being single on Valentine's. For one thing, my mom taught me growing up that the holiday was about all kinds of love, family and friends, not just romantic. Also, it always came across as kind of selfish - seeing your fellow human beings happy should make you happy too, not bitter, that's a miserable way to go about life. So maybe the holiday doesn't offer much to you, but neither does someone else's birthday or an outside culture's holiday, and nobody gets jealous or upset over those. Just because the spotlight isn't on you doesn't mean you can't still give a thumbs up to the people it is on and stay content with yourself; I don't have romantic love, but I still celebrate it as a concept out of acknowledgment of it's importance in the world, and that's always been fine with me. Besides, I wasn't miserable being single the other 364 days of the year and it's not like there's any more couples in existence than usual on Valentine's, so it seems silly to start giving a shit now. Either you're fundamentally upset about being alone or you're not, one day doesn't change that, don't try to make couples feel guilty and complain about a cute little holiday just because you can't decide what to do with yourself.
Basically, have self-esteem and don't be a party-pooper.
Oh, and god that, "Ugh, why do we even have Valentine's Day?! Shouldn't we be showing we love each other ALL THE TIME?!" By that logic, we shouldn't celebrate anything.
"Ugh, why do we even have birthdays?! Shouldn't we be showing our friends and family we're happy they exist ALL THE TIME?!"
"Ugh, why do we even have Christmas?! Shouldn't we be showing generosity and goodwill toward each other ALL THE TIME?!"
"Ugh, why do we even have New Years?! Shouldn't we be setting up goals to better ourselves as people ALL THE TIME?!"
"Ugh, why do we even have Easter?! Shouldn't we be happy that Jesus rose from the dead ALL THE TIME?!"
"Ugh, why do we even have Ash Wednesday or Ramadan?! Shouldn't we practicing self control ALL THE TIME?!"
"Ugh, why do we even have wedding anniversaries?! Shouldn't we be happy that we're married ALL THE TIME?!"
"Ugh, why do we even have the fourth of July? Shouldn't we be proud of being American ALL THE TIME?!"
Like oh gee, just throw your lot in with the Jehovah's Witnesses then already.