I think greeting card companies, including gift shops and flower shops, are not really pushing Valentine's day. They are just trying to exploit the occasion, of course, for a profit. They are just trying to supply products that are high in demand during this occasion. This makes Valentine's day a commercial holiday, if we are to look at it from this perspective. And by the way, this is good for the economy as this spurs economic activity - selling and buying.
Originally Valentine's day is a lethurgical celebration in honor of Saint Valentine who was imprisoned for performing weddings for soldiers who were forbidden to marry during the time of the Roman Empire. Later it became an occasion in which lovers expressed their love for each other by giving gifts, flowers, and other items symbolizing love.
Our need to express our love is so strong that this occasion became one of the most celebrated occasions around the world. And enterprising people find it a great opportunity to make a profit. So the greeting card companies are not really pushing Valentine's day, they are essentially driven by the high demand of products associated with this occasion.