Why do stars tend to stay in a state of equilibrium for most of their lives?

Question

Why do stars tend to stay in a state of equilibrium for most of their lives?

Answer 1

Stars have lots of fuel - hydrogen.  Gravity compresses the fuel which increases the burning rate.  Radiation pressure from the fuel burning expands the star which cools it so as to reduce the burning rate.

Gravity and fuel burning reach equilibrium quickly ( a few hundred million years).  Once this happens there are no major force that would disturb the stability as the star's fuel is slowly used up.