Read the following Greek myth.
Echo and Narcissus
Long ago there lived a handsome man named Narcissus who loved only himself. Echo, a goddess of the mountains, fell in love with him. She followed him everywhere, but he acted coldly and cruelly toward her. She was so sad that she faded away until nothing was left of her but her voice.
Soon after, Narcissus was out taking a walk in the forest. Thirsty, he decided to stop for a drink from a pool. As he leaned down to scoop up the cold water in his hands, he saw himself in the water. The face he saw was so beautiful that he fell in love with it. He could not look away, so he sat and stared at himself in the water. He could not bear to leave, not even to eat or drink.
For the rest of his life, Narcissus never left the pool.
What is the main theme or lesson of the myth?