Why is the troposphere the densest layer?

The troposphere begins at the Earth's surface and extends up to 4-12 miles (6-20 km) high varying from the equator to the poles. At the equator it is around 11-12 miles (18-20 km) high, at 50°N and 50°S, 5½ miles and at the poles just under four miles high. read more

The troposphere is the most dense layer of the atmosphere due to it containing 75% of the atmosphere's mass. And is composed of 78% nitrogen, 21% oxygen, small traces of other gases, and almost all of the atmosphere's water vapor. As the gases in this layer decrease with height, the air becomes thinner. read more