Here's why:

* Gravity: Earth's gravity pulls gases towards the ground, meaning denser gases will sink to the bottom, while lighter gases will rise.

* Temperature: As you move higher in the atmosphere, the temperature generally decreases. This is because the air is less dense, meaning there are fewer molecules to absorb and retain heat.

* Composition: Each layer of the atmosphere has a different composition of gases. For example, the troposphere, the lowest layer, contains the most oxygen and nitrogen, while the stratosphere contains a higher concentration of ozone.

So, the layering of the atmosphere is a result of a combination of gravity, temperature, and composition.