Okay. Greek and Roman mythology: we've got heroes slaying monsters, we've got Zeus throwing lightning, we've got warriors dying gloriously in battle, we've got a guy who thinks his reflection is so hot that he's willing to hang out by the pool and starve to death while he admires himself. Wait, what?
Yeah, we didn't believe it at first either, but there's actually a good reason why stories like Echo and Narcissus exist. While a large portion of mythology exists to record and glorify a culture's history—think The Iliad and The Odyssey—there is no shortage of myths out there that serve as teaching tools. If you've ever read a Bible story or heard a nursery rhyme, you're probably familiar with these teaching stories. In literature, we call them parables.
The first thing that the story of Echo and Narcissus teaches us is to beware the trap of vanity or self-adoration. Basically, don't go around thinking you're all that. Narcissus is so attractive that he practically has to carry around a spear to keep the girls away. The problem is that he knows how attractive he is. He refuses to settle for anyone because he's always confident that there will be others.
When a rejected lover finally cries out for vengeance, her prayers are heard by the goddess Nemesis, who decides to give Narcissus a taste of his own medicine. One glance at his reflection in a pool of water and Narcissus is helplessly captivated. Though he realizes after a while that he's fallen in love with his own reflection, he still can't turn away. He's that impressed with himself.
Not cool, Narcissus. Go sit in the corner by the pool.