Why did Hephaestus leave mount Olympus?

Hephaestus didn't leave Mount Olympus, his mother, Hera, was so ashamed by his ungodly looks that she threw him straight off of Mount Olympus. When Athena was born from Zeus' mind, Hera's jealousy got the better of her. She thought that, if Zeus could make a child just by brains, so could she. The result was Hephaestus, who is told to be the ugliest god there ever was on Mount Olympus.