Let us first think about the color choice. I think that dark red must be the preferred color as it looks like red. And every true lover is ready to shed his/her blood for the beloved. Also love is associated with the heart, and heart pumps blood. So red signifies love. Colors have their own significance, looking at any white color, the first thought is of purity. Same way red means love.
Now let us examine our central theme. Why Rose? A Rose has thorns. But the thorns are below the Rose. The Rose represents life beautifully. It has very soft petals, that tell us about the soft side of living and thorns to convey that life is not all the bed of Roses without thorns. What does a lover do? A true lover will always take the thorns and give only the soft flowers to his/her darling. That is love and love demands that one give only comfort to one's darling. Love demands sacrifice. Love demands giving of joy. Love demands that all pain be kept away from the beloved. That is also the message a lover sends across to his/her beloved with a Rose. I have kept the thorns with myself and I am sending you these soft flowers . I think this is the reason why on Valentine's day, the red Rose is the flower of choice.