Not every pair of atomic energy levels couples equally strongly to light. In the electric-dipole approximation, the transition amplitude between states $|i\rangle$ and $|f\rangle$ is proportional to the matrix element $$\mathbf d_{fi}=\langle f|q\mathbf r|i\rangle.$$ If this matrix element vanishes by symmetry, the electric-dipole transition is forbidden at this approximation even when energy conservation would permit the photon frequency.

For hydrogen-like orbital states, angular integration gives the principal electric-dipole selection rules $$\Delta \ell=\pm1,\qquad \Delta m=0,\pm1.$$ These rules arise from angular momentum and parity of the position operator rather than from an arbitrary empirical restriction.

For example, a $2p\to1s$ transition has $\Delta\ell=-1$ and is electric-dipole allowed. A direct $2s\to1s$ transition has $\Delta\ell=0$ and its electric-dipole matrix element vanishes; the state can still decay through weaker higher-order processes.

'Forbidden' therefore means suppressed in a specified approximation, not impossible under every interaction. Magnetic-dipole, electric-quadrupole, collisions or external fields can enable transitions that the leading electric-dipole coupling excludes.

Selection rules connect symmetry to observable spectra. They explain why spectroscopy contains not every conceivable energy difference, but a structured subset whose strengths encode the quantum numbers and couplings of the states.