I can't think of a single good reason for the to-do. The main beneficiaries are florists and other retailers.
If you're a man with a wife or girlfriend, heaven help you if you don't get the holiday right. Gals, take my word for it: most men hate Valentine's Day. Many are politic enough not to admit it; some are even self-abnegating enough to provide a convincing display of liking it. Any way they look at it or get through it, though, it's still pressure.
If you're a person of either sex who doesn't have a "Valentine" and wish you did, the holiday rubs it in. Worse if if you don't really want the Valentine you have, or at least can no longer think of them in such terms.The only people for whom today is neither pressure nor a bummer are those so in love that they don't need a special day set aside for that. But they get one anyway, and their enjoyment of it does more harm than good for the rest of us—save, of course, for those whose pleasures are primarily vicarious. But you wouldn't want to be one of those.