Traditionally each flower is symbolic and those symbols seem to be firmly in place, even today. In this "language of flowers", red roses can signify passionate love, which is why they are so often given by suitors to the women they are pursuing.
To people who are very sensitive to these meanings, a "misread" can create a bit of havoc. For instance, if you want to say "thank you" to a co-worker and decide to send her a single red rose, you could have a new admirer...:) (Probably better to stick to a nice arrangement of mixed flowers and play it safe.
By the way, you can send the girl you love flowers other than roses. Not many women would say no to a lovely bunch of spring flowers or a bouquet of fall flowers. But when you want to say, "I love only you", nothing does it like red roses.