  We study in this letter the behavior of the neutrinoless double beta decay
nuclear matrix elements (NME's) in the framework of the Interacting Shell
Model. We analize them in terms of the total angular momentum of the decaying
neutron pair and as a function of the seniority truncations in the nuclear wave
functions. This point of view turns out to be very adequate to gauge the
accuracy of the NME's predicted by different nuclear structure models. In
addition, it gives back the due protagonism in this process to the pairing
interaction, the interaction which is responsible for the very existence of
double beta decay emitters. We show that low seniority approximations,
comparable to those implicit in the quasiparticle RPA in an spherical basis,
tend to overestimate the values of the NME's in several decays.
