Venus: Earth's Lost Twin

If you look to the night sky, there is one object (besides the moon) that outshines the rest- our nearest neighbour planet, Venus. Today, Venus is a torrid, toxic planet, and very different to Earth. But the two planets share a similar size, mass and composition, and both are made out of the same materials- and we know think that Venus was once an Earth-like water world for billions of years. So why is Venus such a scorched wasteland today, while Earth has prevailed for billions of years?