Here's why:

* The troposphere is the lowest layer of Earth's atmosphere. It's where we live and where weather occurs.

* It contains about 75-80% of the atmosphere's total mass. This includes the majority of the air we breathe, primarily nitrogen and oxygen.

* The air in the troposphere is denser than in the layers above. This means there are more gas molecules packed into a given space.

As you move higher into the atmosphere, the air becomes less dense and the concentration of gas molecules decreases significantly.