A group of highly mobile particles that exceed the mobility restrictions given by mutually attractive forces among particles cannot mix homogeneously at the microscopic level with a group of low-mobility particles; low-mobility particles that participate in physical cluster formation caused by mutually attractive forces among particles cooperate with high-mobility particles to give the fluid a specific structure. If the pair correlation function is given as the sum of a correlation function that depends on the contribution of particles participating in the physical cluster formation and a correlation function that involves the contribution of high-mobility particles not participating in the physical cluster formation, the behavior of partial contributions to the pressure estimated from the virial equation reveals cooperation generated between particles forming physical clusters and high-mobility particles. Moreover, the behavior of partial contributions to the pressure demonstrates that branches of large stabilized physical clusters can form three-dimensional cavities confining high-mobility particles. The formation of three-dimensional cavities allows a fluid to have a specific microscopic structure.