There are many examples of Greek myths that explain why certain religious rituals were performed, why some peoples may be named what they are, or even why varying objects, plants and animals were symbols of their gods.
The gods were known for punishing mortals for offending them, but occasionally they punished each other. The gods were a vengeful folk, and they did not take kindly to being insulted, by mortal or god. Apollo made the mistake of insulting one of his fellow immortal.
Apollo was a great archer, but sometimes he was a little full of himself. One day he caught sight of Eros, the son of Aphrodite. Eros was also an archer, and his arrows were responsible for instilling the twists and turns of love and lust in a person's heart. Apollo teased young Eros, putting down his abilities as an archer, claiming that one so small could make no difference with his arrows.
Angry at this insult, Eros shot two arrows, one tipped in gold, one blunted and tipped with lead. The arrow dipped in gold had the power to create insatiable lust in a person, while the other created absolute abhorrence towards all things romantic and passionate. The unfortunate soul who was struck with that arrow would have no desire to love anyone. The arrow dipped in gold struck Apollo, but the arrow dipped in lead struck fair Daphne.
Daphne was the daughter of the river god Peneus. Apollo chased down the maiden, desperate for her love, but she wanted nothing to do with him, and she ran from him endlessly. Soon, she grew weary in her running and that Apollo would ultimately catch her. Fearful, she called out to her father for help. As all gods of water posses the ability of transformation, Peneus transformed his daughter into a laurel tree. Suddenly her legs took root, and her arms grew into long and slender branches.
Apollo reached the laurel tree, and, still enamored with Daphne, held the tree in a special place in his heart. He claimed the tree the as his special tree, and adorned himself with some of it's leaves. And that is why the laurel was, and still is, a symbol of the god Apollo.
P.S : quoted passage from http://www.pantheon.org/articles/d/daphne.html