Have you ever wondered why you need to shell out for those long stemmed red roses on Valentines' day? Why can't it be some other equally beautiful, arrangement of red  carnations or white daisies? The next time you wonder whether you should just get a cuddly teddy bear or a box of chocolates, take a note of this fact. Experiments have shown that when women are exposed to flowers, their perception of male attractiveness and their potential mating behavior are affected positively (Gueguen, 2011). Isn't that something! Another study shows that women who received floral bouquets displayed the Duchenne or the true smile more frequently than women who received fruit baskets and also, both men and women showed more positive social behavior when given floral bouquets than when given pens (Haviland-Jones et al, 2005). So then, bringing a bouquet of flowers does have its merits. But why get red roses on Valentines' day? They get so expensive around that time. Well, in a study assessing the effect of different flowers and floral arrangements on a randomly selected group of people, Doyle et al. (1994) have shown that an arrangement of three red roses is particularly appropriate to express "I love you",  "congratulations" or "I apologize" as opposed to yellow carnations, white daisies or blue petunias.
I can almost see the progression of stages from courtship, marriage to  being in the doghouse, guys! But be sure the red rose will stay your companion through all the ups and downs of your journey.

References:

Gueguen N (2011) "Say It with Flowers": The effect of flowers on mating attractiveness and behavior. Social Influence 6(2)-105-111.

Haviland-Jones, J., Rosario, H. H., Wilson, P., & McGuire, T. R. (2005). An environmental approach to positive emotion: Flowers. Journal of Evolutionary Psychology,3, 104–132.

Doyle KO, Hanchek AM, McGrew J (1994) Communication in language of flowers. HortTechnology. 4(3) 211-216.