  The see-saw mechanism of neutrino mass generation may enhance lepton mixing
up to maximal even if the Dirac mass matrices of leptons have structure similar
to that in the quark sector. Two sets of conditions for such an enhancement are
found. The first one includes the see-saw generation of heavy Majorana masses
for right-handed neutrinos and a universality of
  Yukawa couplings which can follow from the unification of neutrinos with new
superheavy neutral leptons. The second set is related to lepton number symmetry
of the Yukawa interactions in the Dirac basis of neutrinos. Models which
realize these conditions have strong hierarchy or strong degeneration of
Majorana masses of the right-handed neutrinos.
