So I don't know what changes, if it's the hormones or living with someone for a while that does this, but Valentine's day is not as Oh LaLa for me as it used to be. That is not to say that I don't enjoy my V-Day with the one I love, I truly do, but I don't get all fluttery like I did when I was in my teens and early twenties.
I could in part be due to hormones not fluctuating so wildly (due to any number of things), or it could be that you and your love one see each other all the time and there is no "wondering" when they will show up with your gift or date. One other thing I have noticed is we all get so caught up in what we have to "get done" that whenever we are together it is "ok good I can get this done because your home now".
There are just so many distractions as we fill out our lives (don't like the term getting older) that we forget that sense of wonder about our lover. As we approach the day of lover's think "My lover is a totally separate person from me, and I will honor that difference because it is what drew me to them", what can you do for them that will make them feel special. When we make others feel good the feelings usually spread.