Q.Why is the force of static friction known as a 'self-adjusting force'?

When an applied horizontal force $F$ acts on a body resting on a surface but the body does not move, the frictional force balancing $F$ is called the force of static friction, $F_s$. Static friction is a self-adjusting force — as $F$ increases from a small value, $F_s$ increases right along with it to keep balancing it exactly, up to a maximum value called the limiting force of friction, $F_L$, reached just before the body starts to slide. The laws of static friction state that $F_L$ is directly proportional to the normal reaction $N$ between the two surfaces, $F_L = \mu_s N$, where $\mu_s = F_L/N$ is the coefficient of static friction; that $F_L$ is independent of the apparent area of contact (for a fixed $N$); and that $F_L$ depends on the materials and surface nature of the two bodies in contact. …