Maybe it's just a hard reality reminder that they don't have a significant other. To me, it's just another day. But to some others, like you said they might have anxiety over loneliness and it just hits them harder with all the reminders of "give the gift s/he'll love" or something all over the place.
People are socially and materially competitive. We want what other people have. If the socioeconomic signal is congruent with the identity that person aspires to have, he or she feels inadequate for not having it.
In other words, being in a relationship is ideal for most people, and Valentine's Day is a test of whether or not you are worthy of having one.
Is it overrated? Yes, but that's how we operate as a species.