Welcome to Pprune, welcome to flying & congratulations on passing air law.
Air is made up of molecules, the amount of which determines its density. Molecules are affected by gravity like everything else. Closer to the Earths surface, lots of them (i.e, dense). Higher up in the atmosphere, fewer of them (i.e, less dense). Density is good for lift, thus more lift closer to the Earths surface.
The Wombat