Why The reflection coefficient, usually written as the Greek letter gamma, is defined by the mismatch between the load and the line: gamma = (ZL - Z0)/(ZL + Z0). It tells you what fraction of the incident voltage wave bounces back from the load, so it is the one parameter that depends on both the load and the feed line. When ZL equals Z0 the numerator is zero, there is no reflection, and SWR is 1:1; SWR and reflection coefficient are directly related by SWR = (1 + |gamma|)/(1 - |gamma|).
Watch out Characteristic impedance is tempting, but it is a property of the transmission line alone, set by its conductor geometry and dielectric, and says nothing about what load is attached. Velocity factor and dielectric constant are also line-only properties describing propagation speed and insulation.
Reflection = interaction. Only the reflection coefficient uses BOTH ZL and Z0: (ZL - Z0)/(ZL + Z0).