Why? It is a holiday designed to sell stuff to people in relationships, and to make people not in relationships feel like shit.
Furthermore: blood diamonds; roses picked by slave labor; chocolate picked by child labor; you see where I'm going with this, right? The "little luxuries" of the holiday are paid for in pain and suffering.
And, of course, the bullshit "give her a diamond and she'll fuck you" script; the "give her an ugly diamond pendant and she'll think you actually put some thought into this crappy holiday" script; the "buy her something at the last minute once a year and you're done being nice for a whole year" script.
Also: the commercial holiday is built on the Catholic holiday that was itself built on top of the Roman holiday of Lupercalia, which wasn't exactly fluffy kittens and sweethearts. More like blood and lust:
The festival was held every year, on the 15th of February,a in the Lupercal, where Romulus and Remus were said to have been nurtured by the she-wolf; the place contained an altar and a grove sacred to the god Lupercus (Aurel. Vict. de Orig. Gent. Rom. 22; Ovid. Fast. II.267). Here the Luperci assembled on the day of the Lupercalia, and sacrificed to the god goats and young dogs, which animals are remarkable for their strong sexual instinct, and thus were appropriate sacrifices to the god of fertility (Plut. Rom. 21; Servius ad Aen. VIII.343).b Two youths of noble birth were then led to the Luperci, and one of the latter touched their foreheads with a sword dipped in the blood of the victims; other Luperci immediately after wiped off the bloody spots with wool dipped in milk. Hereupon the two youths were obliged to break out into a shout of laughter. This ceremony was probably a symbolical purification of the shepherds. After the sacrifice was over, the Luperci partook of a meal, at which they were plentifully supplied with wine (Val. Max. II.2.9). They then cut the skins of the goats which they had sacrificed, into pieces; with some of which they covered parts of their body in imitation of the god Lupercus, who was represented half naked and half covered with goat-skin. The other pieces of the skins they cut into thongs, and holding them in their hands they ran through the streets of the city, touching or striking with them all persons whom they met in their way, and especially women, who even used to come forward voluntarily for the purpose, since they believed that this ceremony rendered them fruitful, and procured them an easy delivery in childbearing. This act of running about with thongs of goat-skin was a symbolic purification of the land, and that of touching persons a purification of men, for the words by which this act is designated are februare and lustrare (Ovid. Fast. II.31; Fest. s.v. Februarius). The goat-skin itself was called februum, the festive day dies februata, the month in which it occurred Februarius, and the god himself Februus.