  A typical form of mass formula is re-explained in terms of nuclear structure.
  For $ N \approx Z $ nuclei, we propose to start with the shell model picture
and to consider the T=0 $2n-2p$ ($\alpha$-like) correlations as the fundamental
concept, instead of the symmetry energy.
  Subsequently, the symmetry energy is described on the basis of the
$\alpha$-like superfluidity caused by the T=0 $2n-2p$ correlations, in parallel
with the pairing energy described on the basis of the pairing superfluidity.
This re-explanation gives useful insight for understanding the nuclear mass
formula. The origin of the Wigner energy is also explained in an interacting
boson model for the Cooper pairs in the $\alpha$-like superfluid vacuum. Adding
a correction term due to the T=0 $2n-2p$ correlations, which determines the T=0
base level for nuclear masses, can improve the mass formulas in practice.
