Venus is the brightest planet in Earth’s night sky because of its highly reflective acidic atmosphere. Over billions of years the atmosphere of Venus has become incredibly thick, likely because of something known as the runaway greenhouse effect, where its volcanic activity contributed to an incredibly dense atmosphere. Such is its thickness now that Venus reflects over 70% of the sunlight that reaches it. Light bounces easily of the clouds, containing sulphuric acid and acidic crystals, high in the atmosphere. This is what makes the planet so bright.
By comparison the Moon reflects only 10% of light that hits it, but because of its proximity to Earth it appears brighter to us.
Image credit: Brocken Inaglory