  The physical origin of the negative thermal correction to the Casimir force
between metals is clarified. For this purpose the asymptotic behavior of the
thermal Casimir force is analyzed at large and small distances in the real
frequency representation. Contributions from propagating and evanescent waves
are considered separately. At large distances they cancel each other in
substantial degree so that only the attractive Lifshitz limit survives. At
smaller separations the repulsive evanescent contribution of s-polarization
dominates in the case of two metals or a metal and a high-permittivity
dielectric. Common origin and order of magnitude of the repulsion in these two
cases demonstrate naturalness of the controversial large thermal correction
between metals.
