  We examine the relationship between individual black hole (BH) masses in
merging binary black hole (BBH) systems. Analyzing the ten BBH detections from
LIGO/Virgo's first two observing runs, we find that the masses of the component
BHs comprising each binary are unlikely to be randomly drawn from the same
underlying distribution. Instead, the two BHs of a given binary prefer to be of
comparable mass. We show that it is $\sim 5$ times more likely that the
component BHs in a given binary are always equal (to within 5\%) than that they
are randomly paired. If we assume that the probability of a merger between two
BHs scales with the mass ratio $q$ as $q^\beta$, so that $\beta=0$ corresponds
to random pairings, we find $\beta>0$ is favored at credibility 0.987. By
modeling the mass distribution, we find that the median mass ratio is $q_{50\%}
= 0.91^{+0.05}_{-0.17}$ at 90\% credibility. While the pairing between BHs
depends on their mass ratio, we find no evidence that it depends on the total
mass of the system: it is $\sim 6$ times more likely that the pairing depends
purely on the mass ratio than on the total mass. We predict that 99\% of BBHs
detected by LIGO/Virgo will have mass ratios $q > 0.5$. We conclude that
merging black holes do not form random pairings; instead they are selective
about their partners, preferring to mate with black holes of a similar mass.
The details of these selective pairings provide insight into the underlying
formation channels of merging binaries.
