Why does Venus have such thick, permanent clouds?

Venus's atmosphere contains sulfur compounds released in the past by volcanic activity, and sunlight drives chemical reactions that produce sulfuric acid. Because the atmosphere is extremely dense, these clouds spread around the planet in a nearly continuous layer. Unlike Earth's water clouds, Venus's clouds sit high in the atmosphere, around 45 to 70 kilometres above the surface. They reflect a lot of sunlight, which is why Venus looks bright from Earth. Even so, the clouds do not cool the surface enough to stop the planet's runaway greenhouse effect.