While I have no real personal experience with romantic love, my understanding of it, inspired by some of the best stories and films I have seen over the years, is that it involves selflessness as its main tenet.
Even the prophetic science fiction novel, The Time Machine, written by H.G. Wells in 1895, and the book that has most inspired me in my life, postulates that even when mind and culture had evaporated in a distant future, selflessness would remain among humanity.
These days, the importance of love has been skewed by the drama of life, but like life itself, it always seems to survive, even in the most inhospitable places.
Romance is more than just a ritual attached to a biological or instinctual necessity to procreate and establish families, but in these difficult, emotion challenged times, it almost seems as if ritual is all that's left. That said, I have observed that romantic love, which is the appreciation of another person more than one's self to a degree that seems all-consuming, remains strong, especially among those whose goals in life supercede the temporal.
Love should be about treating the one you love as more important than yourself. Selflessness is the engine that drives true love. The ultimate expression of love is giving without expecting anything in return, to sacrifice your own joy to instead bring it to the one you love.
So, if you truly love someone, consider their happiness and well being above your own. Do this every day, every moment. True romance is something you must work on all the time, never becoming complacent. Love is like a fire; in order to keep it burning, you have to keep adding fuel. Make her or him feel like they are the center of your life, that they're being alive keeps you alive, that their happiness is the most important over anything else.
Valentine's Day is an annual reminder of what you should be doing every day...loving another person more than yourself, and making it your mission to bring them to joy. Yes, it's true you can't make someone happy as happiness is an individual decision, but you can certainly make it easier for them to arrive at that decision.