The use of red roses as a symbol for love and romance dates back several thousand years. In Greek and Roman mythology, the red rose was associated with the Goddess of Love (in Greek mythology her name was Aphrodite, in Roman it was Venus). Numerous early cultures used red roses as decorations during marriage ceremonies. This helped instill and reinforce the red rose becoming symbolic of--and synonymous with--love and romance around the world.
Although dark red roses are meant as as expression of romantic love, they can also be meant as a sign of deep, nonromantic love and/or admiration. A daughter giving her mother red roses for Mother's Day would be an example of this, as would a lead actress in a Broadway play receiving red roses during a final performance. So in addition to "I love you," red roses can also convey other messages, including congratulations, job well done and respect.