Physics (the particle physics part) is the behavior of elementary particles.

Chemistry is the behavior atoms made of elementary particles interacting with each other via electric charge.

Cell Biology is the behavior of a big stew of hideously complex chemicals in bags of water.

Organismal biology is the behavior of millions of those bags of chemical water all stuck together.

Ecology is the behavior of millions of those organisms interacting across the surface of the earth.

Economics is like ecology, but you are looking at all the intelligent organisms, who are much more behaviorally complex and who occasionally do things like read your economics papers and alter their own behavior in response.

It's no wonder it's complicated!