Why VD is such a big deal? Valentine's Day is a big deal because it is a day set apart for lovers and a woman wants to be remembered by her lover.
Doing something for her on that day shows her that you think of her under the category of lover and that you appreciate that side of her. This may be especially important if you have been married for a while and have several kids.
Valentine's Day is an excuse to bring back some of the fire.