  We demonstrate that the polarization fractions of most tree-dominated $B\to
VV$ decays can be simply understood by means of kinematics in the heavy-quark
or large-energy limit. For example, the longitudinal polarization fractions
$R_L$ of the $B^0\to (D_s^{*+}, D^{*+}, \rho^+)D^{*-}$ and $B^+\to (D_s^{*+},
D^{*+}, \rho^+)\rho^0$ modes increase as the masses of the mesons $D_s^{*+},
D^{*+}, \rho^+$ emitted from the weak vertex decrease. The subleading
finite-mass or finite-energy corrections modify these simple estimates only
slightly. Our predictions for the $B\to D_{(s)}^* D^*$ polarization fractions
derived in the perturbative QCD framework, especially $R_L\sim 1$ for $B^0\to
{\bar D}^{*0} D^{*0}$ governed by nonfactorizable $W$-exchange amplitudes, can
be confronted with future data. For penguin-dominated modes, such as
$B\to\rho(\omega) K^*$, the polarization fractions can be understood by the
annihilation effect from the $(S-P)(S+P)$ operators, plus the interference with
a small tree amplitude. At last, we comment on the various mechanism proposed
in the literature to explain the abnormal $B\to \phi K^*$ polarization data,
none of which are satisfactory.
