  We investigate analytically the anisotropic dielectric properties of single
crystal {\alpha}-SnS near the fundamental absorption edge by considering atomic
orbitals. Most striking is the excitonic feature in the armchair- (b-) axis
direction, which is particularly prominent at low temperatures. To determine
the origin of this anisotropy, we perform first-principles calculations using
the GW0 Bethe-Salpeter equation (BSE) including the electron-hole interaction.
The results show that the anisotropic dielectric characteristics are a direct
result of the natural anisotropy of p orbitals. In particular, this dominant
excitonic feature originates from the py orbital at the saddle point in the
{\Gamma}-Y region.
