Eos, the Goddess of Dawn, was a dewy young daughter of the Sun. Like other young maidens, she fell in love with beautiful young men, and Eos had a thing for pretty young mortal men. The primary object of her affections was a a handsome young Trojan named Tithonius.

Eos really liked him a lot - she had his son, Memnon, and made Memnon the King of Ethiopia. But Tithonius was content to just sit and chill with Eos. The problem came when the couple realized that Tithonius, being mortal, would age. So Tithonius begged Eos for immortality, and Eos begged Zeus. Zeus granted the couple's wish - despite his reservations about the idea (but he couldn't say anything, because he slept with mortals all the time).

Tithonius was indeed granted immortality - but the two did NOT live happily ever after. Here's why: Tithonius had been granted eternal life - that's true - but NOT eternal youth. So Tithonius grew older and older and more and more unattractive until he shrunk into a grasshopper. Eos was completely disgusted and couldn't deal with his constant chirping or his ugly form, so she let him into the world.

Another story says that when he got really really old he begged Eos for death - but once immortal, there's no changing things back, so she turned him into a grasshopper instead.