  We discuss an anthropic explanation of why there exist three generations of
fermions. If one assumes that the right-handed neutrino sector is responsible
for both the matter-antimatter asymmetry and the dark matter, then anthropic
selection favors three or more families of fermions. For successful
leptogenesis, at least two right-handed neutrinos are needed, while the third
right-handed neutrino is invoked to play the role of dark matter. The number of
the right-handed neutrinos is tied to the number of generations by the anomaly
constraints of the $U(1)_{B-L}$ gauge symmetry. Combining anthropic arguments
with observational constraints, we obtain predictions for the $X$-ray
observations, as well as for neutrinoless double-beta decay.
