A rose is a rose is a rose (Gertrude Stein, 1913)

 

While many of us link roses with romance and love, this wasn't always so. In fact, this gorgeous bloom was associated with one of the bloodiest conflicts of 15th century England. The Wars of the Roses were a series of wars for control of the throne of England fought between supporters of two rival branches of the royal House of Plantagenet: the House of Lancaster (associated with a red rose), and the House of York (whose symbol was a white rose). This 30-year feud finally ended with the death of Richard III at the Battle of Bosworth.
Below, from Carlton Hobbs:
A fine floral painted KPM circular porcelain plaque signed KRÜGER FECIT 1819. Berlin, 1819.

Nowadays, long-stemmed red roses are practically synonymous with Valentine’s Day. This tradition of giving Valentine’s Day flowers dates back to the late 17th century, during the reign of King Charles II of Sweden. During a trip to Persia, King Charles II was exposed to a new art—the language of flowers, the ability to communicate using flowers without uttering any words at all. The fad boomed throughout Europe, with lists of flowers and their meanings widely distributed. The red rose translated of course to deep love.
Below, from Butchoff Antiques:
A five-fold screen in the early romantic manner. French, circa 1880.

The rose took on this meaning because of its affiliation with Aphrodite, the goddess of love. In Greek mythology, it is said that rose bushes grew from the ground through Aphrodite’s tears and the blood of her lover, Adonis. The Romans, who turned Aphrodite into their goddess Venus, kept the rose as her symbol of love and beauty. So when Valentine’s Day became the mainstream holiday we know today, the rose was an obvious choice for the most fitting gift.
Below, from Philadelphia Print Shop West:
Rosa Ferox and Rosa Indica, from Roses: or Monograph of the Genus Rosa. London, 1805. Engraved and hand colored by H.C. Andrews.

Did you know that there are symbolic meanings associated with other rose colors as well? Yellow roses send a message of appreciation and platonic love without the romantic subtext of other colors. Shades of lavender roses suggest an air of regal majesty and splendor. Dark pink roses are symbolic of gratitude and appreciation, and are a traditional way to say thanks. Light pink roses are associated with gentleness and admiration, and can also be used as an expression of sympathy. 
Below, from Earle D. Vandekar of Knightsbridge Inc.:
German Porcelain Trompe L'oeil Rose Tea Service, 19th Century. 

Also known as the bridal rose, the white rose is a traditional wedding flower. In this sense, white represents unity, virtue, and the pureness of a new love. White roses are also associated with honor and reverence, which makes them a fitting memorial for a departed loved one. Finally, orange roses – being a mixture of yellow and red - were seen as a bridge between friendship symbolized by yellow roses and love represented by red roses.
Below, from Haynes Fine Art:
Shades of Spring by Marcel Dyf
Oil on canvas, signed lower right. French 1899-1985.