Field lines relax(where they can)

Let an elastic cord go, and it relaxes by getting shorter and maybe straighter. Field lines do the same: relax by shortening or straightening.

Compare two situations. If such a relaxing is possible, then there is process linking the situations that goes by itself. If the field lines cannot relax, then there will need to be some must be “exerting by” and “acting on” to make the process go.

Put another way: if the field lines relax, the magnetic field provides a force. If the field lines don’t relax, then a force has to be exerted from an external environment.