Population Inversion

Atoms are lazy: they don't like to stay in an excited (or high energy) state.
It is tiring. They prefer to relax into a lower energy) state and enjoy life!

If we don't have a lot of excited atoms to start with, the chance of having a lot of stimulated emission is not great!

To get a laser to work, we need to have a population inversion: we need to get most of the electrons to the high energy states.
This is done by providing an initial energy to the atoms: passing electrical current, illuminating with a bright pulse, ...