Why did the gods make Narcissus fall in love with his own reflection?

Variation One:

Narcissus is the son of the river god Cephissus and the nymph Liriope. Tiresias, the seer, told his parents that the child "would live to an old age if it did not look at itself." Many nymphs and girls fell in love with him but he rejected them. One of these nymphs, Echo, was so distraught over this rejection that she withdrew into a lonely spot and faded until all that was left was a plaintive whisper. The goddess Nemesis heard the rejected girls prayers for vengeance and arranged for Narcissus to fall in love with his own reflection. He stayed watching his reflection and let himself die.

Variation Two:

An earlier variation of this tale originates in the region in Greek known as Boeotia (to the north and west of Athens). Narcissus lived in the city of Thespiae. A young man, Ameinias, was in love with Narcissus, but he rejected Ameinias' love. He grew tired of Ameinias' affections and sent him a present of a sword. Ameinias killed himself with the sword in front of Narcissus' door and as he died, he called curses upon Narcissus. One day Narcissus fell in love with his own reflection in a spring and, in desperation, killed himself.

Both of these stories give an origin to the narcissus flower, which grew where Narcissus died.