Narcissus was a beautiful nymph, all the other nymphs had loved his looks.
One day, he had told all the people that no one can beat his beauty.
When Apollo had heard about this he had raged, he did not want the gods to be forgotten for just one nymph.
I do not believe you, and you might have just said this because I am beautiful.
One day, a young boy had said that someone is more beautiful than Narcissus.
One day, Apollo had came down to Narcissus, and said that he himself was more beautiful, and was the beautiful god. Narcissus could not believe this, and so he had asked the god to prove he was a god.
Narcissius was amazed, so he had loved seeing the Gods. He had said that he would never say he is more beautiful than anyone.