Complexation in symmetric solutions of oppositely charged polyelectrolytes is studied theoretically. We include polyion cross-linking due to formation of thermoreversible ionic pairs. The electrostatic free energy is calculated within the random phase approximation taking into account the structure of thermoreversible polyion clusters. The degree of ion association is obtained self-consistently from a modified law of mass action, which includes a long-range electrostatic contribution. We analyze the relative importance of the three complexation driving forces: long-range electrostatics, ion association, and van der Waals attraction. The conditions on the parameters of the system that ensure stability of the complex with addition of salt are determined.