Venus' current environment is the result of a "greenhouse effect" that has allowed its massive CO2 atmosphere to intake the heat of the sun and retain that heat over time like a greenhouse for flowers here on Earth.
Were it not for this greenhouse effect, even with Venus' location closer to the sun by about 25% relative to our Earth (which means Venus receives twice as much energy from the sun as Earth), its surface and atmosphere may have been more Earth-like in the past, with climate models suggesting that it could have been more clement, i.e., 50–80 degrees C (122–176 F) rather than 440-460 degrees C (824–860 F).
How Venus developed this greenhouse effect so prominently is a key question about our sister planet. It is possible that sustained volcanism on a planet that lost its surface oceans of liquid water allowed for more heat energy from active eruptions to also heat the atmosphere from below, influencing the thick deck of clouds that serves today to keep the heat deep in Venus's atmosphere. On Earth, carbon dioxide outgassed by volcanoes is recycled back into the planet's interior on geological timescales through plate tectonics; on Venus, with no plate tectonics, carbon dioxide remains in the atmosphere as a robust greenhouse gas.