This may seem a silly question (and possibly is) but why do roses have thorns? Why do nettles sting you? Presumably these are both self-defence mechanisms, but as they have been around long before humans appeared on the scene - self-defence against what?
Cupid actually gave the rose its thorns. One day, Cupid stopped to smell a rose. Hidden inside of that rose there was a bee which shot out and stung him. Exasperated, he pulled out an arrow and shot it right into the flower in anger, resulting in the rose everafter sprouting thorns.